Lesson: Percent Increases and Decreases

Comment on Percent Increases and Decreases

Great video! Just letting you know the first related resource says Manhattan Prep but links to Gmat Club!
gmat-admin's picture

Thanks! It's actually both :-)
It's a question on GMAT Club that's created by Manhattan Prep. The questions listed as GMAT Club are questions created by the people at GMAT Club.

This question seems ambiguous to me. I assumed the reference to the net result was to Hilda's profits, meaning the answer would be 18% (her profits are the same as if she had sold at regular price after paying $118 for the product). Who cares what % decrease the sale price is compared to the wholesale price.
gmat-admin's picture

You're referring to the question that starts at 3:45.

I can see how "net" could cause confusion, since the word has more than one meaning. In this case, "net" refers to the combined effect of the price increase and decrease. I believe we'd have to add a word like "profit" in order to convey the meaning that you're suggesting.

Point taken though - thanks!

maybe another way to think about it

1*1.2*0.85 = 1.02

therefore 2% increase

gmat-admin's picture

Yes, that's basically what we're doing in the video. Except we're starting with 100 rather than 1.

How do you know or go about determining the difficulty levels for the practice problems? Will the harder ones only appear if you continuously get the previous problems correct?
gmat-admin's picture

Are you're referring to what happens during the official practice tests (the GMATPrep tests)?

On the official practice tests (and on test day), the questions you see are based on your success rate on previous questions. So, if you keep answering questions correctly, you will see harder and harder questions.

Conversely, if you keep getting questions wrong, then you'll be given easier and easier questions.

For more on the GMAT's scoring algorithm, watch https://www.gmatprepnow.com/module/general-gmat-strategies/video/1251

Sorry let me rephrase using this as an example: 'GMAT practice question (difficulty level: 650 to 800) - Manhattan Prep'

How would you know this is a 650-800 level GMAT quant difficulty question?
gmat-admin's picture

Those categorizations are my best guess at the difficulty levels. I've been teaching the GMAT for quite a while, so I have a good idea of what kinds of questions cause difficulties for students. Also, the practice questions in the Official Guide for GMAT Review are ordered in ascending difficulty, which also helps me gauge difficulty levels.

Of course, I can't be super accurate and say a question is a 590-level question, which is why I provide a range.

Hi Brent,

Please help me with the solution:

Of the 3,600 employees of Company X, 1/3 are clerical. If the clerical staff were to be reduced by 1/3 what percent of the total number of the remaining employees would then be clerical?
(A) 25 %
(B) 22.2 %
(C) 20 %
(D) 12.5 %
(E) 11.1 %
gmat-admin's picture

If 1/3 of the 3,600 employees are clerical, then there are 1200 clerical workers.

If 1/3 of the clerical staff are laid off, then 400 of the 1200 clerical workers are laid off.

This means there are 800 clerical workers remaining.

Since 400 workers were laid off, the NEW employee population = 3600 - 400 = 3200

"What percent of the total number of the remaining employees would then be clerical?"

Of the 3200 workers, 800 are clerical workers

800/3200 = 1/4 = 25%

Answer: A

The cost C of manufacturing a certain product can be estimated by the formula C = 0.03 rst², where r and s are the amounts, in pounds, of the two major ingredients and t is the production time, in hours. If r is increased by 50 %, s is increased by 20 %, and t is decreased by 30 %, by approximately what percent will the estimated cost of manufacturing the product change ?

(A) 40% increase
(B) 12% increase
(C) 4% increase
(D) 12% decrease
(E) 24% decrease
gmat-admin's picture

Let's plug in some nice values of r, s and t, and then see what happens when we make the given changes.

Let's try r = 10, s = 10 and t = 10

So, cost = 0.03rst² = 0.03(10)(10)(10²) = 3000

If r is increased by 50 %, then r = 15
If s is increased by 20 %, then s = 12
If t is decreased by 30 %, then t = 7
So, NEW cost = 0.03rst² = 0.03(15)(12)(7²)

Let's use some ESTIMATION....

= (0.03)(180)(7²)
= (3/100)(180)(7²)
= (540/100)(7²)
≈ (5.5)(7²)
≈ (5.5)(50)
≈ 275

So, the original cost was $300 and the new cost is $275

This is a DECREASE, so eliminate A, B and C

Percent change = (100)(change)/original
= (100)(300 - 275)/300
= 25/3
≈ 8 1/3%
≈ 8.333%

The closest answer is D, so it must be correct.

Dear Brent,

Thank you for the reinforcement activity questions. They represent a great pool of questions for various difficulty levels. Though I was able to solve (and understand the explanation for) most of the questions above, this particular question perplexed me and I am still unable to understand the explanations provided underneath the question. Kindly help me understand it in a better and simpler manner if possible. Thanks.

Here's the question for your reference:

A certain salesman's yearly income is determined by a base salary plus a commission on the sales he makes during the year. Was the salesman's commission larger than his base salary last year?

(1) If the amount of the commission had been 30 percent higher, the salesman's total income (salary plus commission) would have been 10 percent higher last year.

(2) The absolute difference between the amount of the salesman's base salary and the amount of the commission was equal to 50 percent of the salesman's base salary last year.

Thanks and Regards,
Aashay
gmat-admin's picture

Glad to help.
Here's my step-by-step solution: http://www.beatthegmat.com/income-t285243.html#798714

Cheers,
Brent

Hi Brent, Could you please help me solve this question by assuming some values?

The annual rent collected by a corporation from a certain building was x percent more in 1998 than in 1997 and y percent less in 1999 than in 1998. Was the annual rent collected by the corporation from the building more in 1999 than in 1997?
(1) x > y
(2) xy/100 < x - y
gmat-admin's picture

There's a nice discussion (by several experts) here: http://www.beatthegmat.com/og-13-127-t288372.html

If you have any questions about those approaches, I'm happy to help.

Cheers,
Brent

Sir is thera any other easy way to remember the question

By approximately what percent is x greater than 4/5 if (4/5)(x) = 1?

A. 73%
B. 56%
C. 41%
D. 37%
E. 29%
gmat-admin's picture

Here's one approach:

Given: (4/5)(x) = 1
Divide both sides by 4/5 to get: x = 5/4

So, one option is to determine by what percent 5/4 is greater than 4/5
However, working with fractions can be a pain, so we could also go this route:
5/4 = 25/20 and 4/5 = 16/20

Since both values have the same denominator, we can determine by what percent 25 is greater than 16 (a little easier)
This is the same as the percent increase from 16 to 25

Percent increase = (100)(new - old)/old
= (100)(25 - 16)/16
= (100)(9)/16
≈ 56%

Answer: B

https://gmatclub.com/forum/each-year-for-4-years-a-farmer-increased-the-number-of-trees-in-a-135487.html

Hi Brent,
I find it difficult to solve this problem. I am not clear with the successive percent change calculations. Could you please help me out? Also if you have answered this question elsewhere please share the link.

Thanks in advance.
gmat-admin's picture

Here's my solution: https://gmatclub.com/forum/each-year-for-4-years-a-farmer-increased-the-...

Please let me know if you'd like me to elaborate on any parts of my solution.

Cheers,
Brent

A merchant discounted the sale price of a coat and the sale price of a sweater. Which of the two articles of clothing was discounted by the greater dollar amount?

(1) The percent discount on the coat was 2 percentage points greater than the percent discount on the sweater.
(2) Before the discounts, the sale price of the coat was $10 less than the sale price of the sweater.

https://gmatclub.com/forum/a-toy-store-regularly-sells-all-stock-at-a-discount-of-103585.html

A toy store regularly sells all stock at a discount of 20 percent to 40 percent. If an additional 25 percent were deducted from the discount price during a special sale, what would be the lowest possible price of a toy costing $16 before any discount?

A. $5.60
B. $7.20
C. $8.80
D. $9.60
E. $15.20

Hi Brent, could you please offer some insight for this question.

Firstly, the initial approach I took to solve this question was that in order to reverse the discounts, you would have to multiply the discounted price by (1+Discount %). I.E. = 16 * (140/100) * (125/100).
This implying that the INITIAL PRICE was reduced by 40%, then 25%. However this approach is wrong and I am not sure why.

The second part of this question which confuses me are the answer choices. The question stem specifically states:

"What would be the lowest possible price of a toy costing $16 before any discount?"

IF the cost is $16 after discount, how could it possibly be any of these answer choices??
A. $5.60
B. $7.20
C. $8.80
D. $9.60
E. $15.20

IF we are trying to find the cost of the Toy "before" discount, this price we find would be higher than $16.

Thank you and apologies if I overlooked anything and made a simple error.

gmat-admin's picture

This is a bit of a Sentence Correction question.

It all comes down to the interpretation of the line "What would be the lowest possible price of a toy costing $16 before any discount?"

Notice that "before any discount" is NEXT TO "costing $16"
This tells us that the before-discount cost is $16
So, the after-discount price will be less than $16

You're reading the line as "Before any discounts, what would be the lowest possible price of a toy costing $16?"
In this case, the answer would be greater than $16

Does that help?

Here's my solution: https://gmatclub.com/forum/a-toy-store-regularly-sells-all-stock-at-a-di...

Cheers,
Brent

Hi Brent, could you please help to provide solution to this DS question:

Jason’s salary and Karen’s salary were each p percent greater in 1998 than in 1995. What is the value of p ?

(1) In 1995 Karen’s salary was $2,000 greater than Jason’s.
(2) In 1998 Karen’s salary was $2,440 greater than Jason’s.

Thanks!
gmat-admin's picture

You bet!
Here's my full solution: https://gmatclub.com/forum/janson-s-salary-and-karen-s-salary-were-each-...

Cheers,
Brent

Company P had 15 percent more employees in December than it had in January. If Company P had 460 employees in December, how many employees did it have in January?

(A) 391
(B) 400
(C) 410
(D) 423
(E) 445

Thank you Brent for explaining the common mistake. I would like to know how did you get to 1.15.... please explain the rule because I want to understand it

Thank you for your prompt help
Fatima-Zahra
gmat-admin's picture

My solution: https://gmatclub.com/forum/company-p-had-15-percent-more-employees-in-de...

If we let J = # of employees in January, then...

# of employees in Dec = (number employees in Jan) + (15% of the number employees in Jan)
= (J) + (15% of J)
= J + 0.15J
= 1.15J

Let's examine one more example.

y is 37% greater than x
We can write: y = x + (37% of x)
y = x + 0.37x
y = 1.37x

Does that help?

Cheers,
Brent

Martha's Hair Saloon has recently lowered the prices of haircuts. If the decrease in the price of a haircut is 20% of the new price of a haircut, what is the approximate percent decrease in the price of a haircut at Martha's ?

A. 15%
B. 16.7%
C. 20%
D. 25%
E. 87.5%

Hi Brent, could you help me out in this question with the input/ output method please? Thank you before hand!
gmat-admin's picture

The INPUT-OUTPUT method is used when we have a Variables in the Answer Choices question (aka VIAC).
So, that approach wouldn't work here.

For more on VIAC's, watch: https://www.gmatprepnow.com/module/gmat-word-problems/video/933

Here's my full solution: https://gmatclub.com/forum/martha-s-hair-saloon-has-recently-lowered-the...

Cheers,
Brent

Hi Brent,

question link https://gmatclub.com/forum/the-output-of-a-factory-was-increased-by-10-to-keep-up-with-138775.html

So I am a little confused with formula % Change=(new-original)/original. Am I understanding it correct that this formula works when we are trying to calculate percent INCREASE. When we are going the opposite direction and are trying to calculate the percent decrease from a number than instead of dividing difference by original number, we must divide difference by new number (in case of this question we are trying to calculate by how much percent we have to decrease a number (say original number is 100 and final number is 132), it is the difference in relation to 132, not 100. So we are dividing 32/132. Am I getting the concept correct? Otherwise, I do not understand why we are dividing 32 by 132, not the 100.

Thanks a bunch!
gmat-admin's picture

Question link: https://gmatclub.com/forum/the-output-of-a-factory-was-increased-by-10-t...

In general, percent increase or percent decrease = (100)(change)/(old value)

This question is somewhat confusing, since we already increased the output twice, and now we're trying to get back to the original value.

However, our goal is to go from 132 to 100.
So, 132 is the OLD value, and 100 is the NEW value.

So, percent decrease = 100(132-100)/132

Does that help?

Cheers,
Brent

hi Brent, can you please explain Fluke's approach in this question https://gmatclub.com/forum/if-7-workers-can-build-7-cars-in-7-days-then-how-many-days-would-it-112832.html .
gmat-admin's picture

Question link: https://gmatclub.com/forum/if-7-workers-can-build-7-cars-in-7-days-then-...

The question: If 7 workers can build 7 cars in 7 days, then how many days would it take 5 workers to build 5 cars?
Here's Fluke's solution, with my comments in the brackets

7 Worker - 7 cars - 7 days [given]
1 Worker- 7 cars - 7*7 days [since 1 worker must do the job of 7 workers, it will take that 1 worker 7 TIMES as long to complete the job]
1 Worker - 1 car - 7*7/7=7 days [if we reduce the job from making 7 cars to making 1 car, the work time will be 1/7 of the former work time]
1 Worker - 5 car - 7*5 days [if we multiply the number of cars needed by 5, then we multiply the work time by 5]
5 Worker - 5 cars - 7*5/5 = 7 days [if we multiply the number of workers by 5, then the work time is divided by 5]

Does that help?

Cheers,
Brent

Hi Brent,

Could you please help with the following question?

The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present. If the concentration of chemical B is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

100% decrease

50% decrease

40% decrease

40% increase

50% increase

Thanks,
Kevin
gmat-admin's picture

Tough question!!!

From the given information, we can write: Rate of reaction = A²/B

Let's plug in some INITIAL values.
Let's say the concentration of chemical A present = 6
Let's say the concentration of chemical B present = 2
With these values, the rate of reaction = 6²/2 = 36/2 = 18

Now let's see what happens when the concentration of chemical B is increased by 100 percent.
This means the concentration of chemical B increases to 4
So, the rate of reaction = A²/4
We want the rate to REMAIN at 18
So, we get the equation A²/4 = 18

Solve for A by first multiplying both sides by 4 to get: A² = 72
Solve: A = √72
So, in order for the rate to stay at 18, A must equal √72

We know that √64 = 8 and √81 = 9
Since 72 is roughly halfway between 64 and 81, we can conclude that √72 is roughly halfway between 8 and 9
So, let's say A = 8.5

So, A must increase from 6 to 8.5

Percent increase = 100(8.5 - 6)/6
= 100(2.5)/6
= 250/6
≈ 40%

Answer: D

Cheers,
Brent

Hi Brent,

Thanks for the fast and detailed solution!

What got me confused was the term "concentration" not referring to a percentage.
Is it used for percentages as well as any other number regularly in the GMAT or is this question an exception?

Cheers,
Kevin
gmat-admin's picture

Good question, Kevin.

Concentration can be measured in many different ways (e.g., by percent, parts per million, grams per liter, etc).
On some occasions, the GMAT test-makers will provide units of measurement, and on other occasions, they don't.

For example, with Geometry questions, the test-makers will often drop the units of measurement and just say "some line has length 10" or "rectangle R has area 6", etc. Other times, they'll include units, as in "the flask contains 3 gallons of water."

Cheers,
Brent

Thank you for the explanation and examples!

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