Lesson: Solving GMAT Age Questions

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Assuming the question changes to 4 years ago, Soo was twice as old as Marco. Will the equation change to 1/2(M+4)?

I was working on this question here (http://www.beatthegmat.com/equations-t269315.html#691952) and the age translation changed. Does the past and future change whether equation translation?
gmat-admin's picture

If the question were worded so that it gave information about their ages 4 years AGO, then...
4 years ago, Marco's age was M-4
And 4 years ago, Soo's age was M+4
Since Soo was twice as old as Marco, we need to DOUBLE Marco's age to make the ages equal.
So, M+4 = 2(M-4)

I dont understand why you are doubling marcos age?
gmat-admin's picture

We're told that Soo is twice as old as Marco.

Our goal is to create an EQUATION. However, at the moment, we cannot say that their ages are equal since Soo's age is much bigger than Marco's age.

So, how can we make their ages equal?

We can take the smaller age (Marco's age) and double it so that it equals Soo's age.

More here: https://www.gmatprepnow.com/module/gmat-word-problems/video/903

So, I go:
Marco = M
Soo = M+8
In 4 years,
Soo = (M+8)+4
Marco = M+4
Given:
M+8+4 = 2(M+4)
M=4
Soo's age = 4+8 = 12
gmat-admin's picture

Perfect!

Hey Brent,
Just a question: Is this question really a possible question we could see on the GMAT considering how long it generally takes to solve it? Or is it just to reinforce concepts.
"In three years, Janice will be three times as old as her daughter. Six years ago, her age was her daughter’s age squared. How old is Janice?"

Also I found a decent way to solve to question really fast.

Since we know that the answer choices are all Janice's age, and that Janice's age 6 years ago was a perfect square. I subtracted 6 from all answer and only one of them yielded a perfect square (42 -6 = 36).
In the event another answer also yielded a perfect square, we can test her daughters age 6 (root of 36) by adding 9 (6 years ago + 3 years hence) = 6 + 9 = 15.
Also Janice's age 42 + 6 = 45. This satisfies the condition of Janice's age 3 years hence being thrice the daughters age. Verifying the fact that we have the correct answer.

Thanks !
gmat-admin's picture

Here's the link to your question: https://gmatclub.com/forum/in-three-years-janice-will-be-three-times-as-...

I think all of the linked questions in the Reinforcement Activities box are representative of the GMAT.

Your solution is PERFECT!! In fact, I have posted it here (with kudos to you, of course!): https://gmatclub.com/forum/in-three-years-janice-will-be-three-times-as-...

Hi Brent

In the single variable explanation of the Abbie Iris question. Can you always assume the smallest number number = the variable. The reason I ask this is that it seems that you miss a step if you go from Iris age=I and therefore Abbie's age = 42-I. Why would you not write Iris's age in terms of the sum of 42 as well?

That is where I got bogged down. I tried to use the sum as my reference, therefore Abbie Age + Iris Age = 42, therefore Iris Age = 42 - Abbie age and Abbie age = 42 - Iris's age.

This did led me down the wrong path. Any tips?

Kind regards
gmat-admin's picture

Great question, Willem!

I find it easier to assign the single variable to the smaller age. That way, we can often avoid using fractions and negatives.

That said, we could have just as easily assigned the variable to Abbie's PRESENT age. Let's do that.

Let A = Abbie's PRESENT age
So, 42 - A = Iris's PRESENT age

11 years ago, each person was 11 years YOUNGER. So...
A - 11 = Abbie's age 11 YEARS AGO
So, 42 - A - 11 = Iris's age 11 YEARS AGO

Simplify to get:
A - 11 = Abbie's age 11 YEARS AGO
So, 31 - A = Iris's age 11 YEARS AGO

GIVEN: 11 years ago, Abbie was three times as old as Iris.

In other words:
(Abbie's age 11 YEARS AGO) = 3(Iris's age 11 YEARS AGO)
We get: A - 11 = 3(31 - A)
Expand: A - 11 = 93 - 3A
Add 3A to both sides: 4A - 11 = 93
Add 11 to both sides: 4A = 104
Solve: A = 26

So, Abbie's PRESENT AGE is 26

The question asks for Abbie's age in 2 years.
Answer = 26 + 2 = 28

Does that help?

Cheers,
Brent

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