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- General GMAT Strategies - 7 videos (all free)
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## Comment on

Introduction to Ratios## Awesome thanks for the lesson

## Ratios are important, but I

Ratios are important, but I wouldn't say they're tested as often as the super popular concepts like percents, geometry, statistics, integer properties and overlapping sets.

## I always mix up the ratios

## This is a common error.

This is a common error.

To help avoid making this mistake, it's useful to jot down the order in which you are presenting your info. Here's what I mean.

Let's say we're told that the ratio of girls to boys is 2 to 3.

Some students will take this information and write 2 : 3 (or 2/3) on their notepad. The problem with this is that it isn't clear what the two numbers represent.

Instead, I suggest that you first write: girls/boys to help you keep track of what each number represents. Then, when you write 2/3, you will be less likely to mix up the values.

You'll see that I do this throughout the videos.

## In a certain district, the

(A) 100

(B) 300

(C) 400

(D) 1,000

(E) 2,500

. Please help can't figure out .

## There are some nice

There are some nice approaches to that question here: http://www.beatthegmat.com/ratio-shorter-method-t114168.html and here https://gmatclub.com/forum/in-a-certain-district-the-ratio-of-the-number...

Let me know if you'd like me to solve as well.

## I solved this problem and I

Solution:

Let X = Registered Republicans

Let Y = Registered Democrats

X/Y=3/5

X=(3/5)Y equation (1)

After Additional:

(X+600)/(Y+500)=4/5 Using (a/b)=(c/d) -> ad=bc

5(X+600)=4(Y+500)

5X+3,000=4Y+2,000 equation (2)

Replacing equation (1) in equation (2)

5[(3/5)Y]+3,000=4Y+2,000

3Y+3,000=4Y+2,000

Solving: Y=1,000

Putting Y in the equation (1): X = (3/5)*1,000

X=600

Y-X=1,000-600

Y-X=400

Help me to find what I'm doing wrong.

Thanks,

Pedro

## Everything is perfect . . . .

Everything is perfect . . . . . except we're not done yet.

You determined that X = 1000 and Y = 600.

However, X = Registered Republicans ORIGINALLY, and Y = Registered Democrats ORIGINALLY.

The question asks, "AFTER these registrations, there were how many more voters in the district registered as Democrats than as Republicans?"

So, AFTER the registrations...

X + 500 = number of Democrats, and...

Y + 600 = number of Republicans

So, AFTER the registrations...

1000 + 500 = 1500 = number of Democrats, and...

600 + 600 = 1200 = number of Republicans

So, there are 300 more Democrats than Republicans

## Wow, I forgot to sum after

Thank you.

## Hey, thanks for all the

## Here's another way to look at

Here's another way to look at the a/b notation.

Rather than say the ratio a/b represents "a out of b", we can say "For every b things there are a things"

For for example, if 2/3 of the students are girls, we can say "For every 3 students, 2 are girls"

In this case, the ratio of girls to total students is 2:3 (or 2/3)

Does that help?

Cheers,

Brent

## For the reptiles and birds

## Good question.

Good question.

Cross simplifying only works when we are multiplying fractions.

However, cross MULTIPLYING is something completely different.

If 7/2 = R/28, then we can solve for R by first cross multiplying to get: (2)(R) = (7)(28), and then solving this equation for R

Cheers,

Brent

## Also for the cookies and nuts

## Your approach is perfectly

Your approach is perfectly valid.

I start by drawing bags/buckets of values to show why the technique works. I imagine many students, once they're familiar with the related concepts, gradually transition away from drawing bags.

## Hi Brent, can you please

Yesterday, the ratio of males to females working at Gigacorp was 2:3. Today, Gigacorp hired an additional 81 employees, and the ratio of males to females is now 6:5 (no employees left or were fired). What is the least number of males that could have been working at Gigacorp yesterday?

A) 16

B) 24

C) 39

D) 40

E) 42

## Hi Jalal,

Hi Jalal,

Here's my step-by-step solution: https://gmatclub.com/forum/yesterday-the-ratio-of-males-to-females-worki...

Cheers,

Brent

## Add a comment