Mother and daughter

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AIM TO CRACK GMAT: Senior | Next Rank: 100 Posts; Posts: 80; Joined: Sat Sep 15, 2012 1:07 amFollowed by: 1 member; Thanked: 1 times

by AIM TO CRACK GMAT » Mon Oct 21, 2013 2:06 pm

A Mother's age after 6 years will be thrice the age of her daughter now. The sum of their ages 6 years ago was 42. What is the ratio of the current ages of the mother and the daughter?

(A) 18:7 (B) 7:5 (C) 6:1 (D) 13:5

D

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Source: Beat The GMAT — Problem Solving | Mon Oct 21, 2013 2:06 pm

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Mon Oct 21, 2013 2:26 pm

AIM TO CRACK GMAT wrote:A Mother's age after 6 years will be thrice the age of her daughter now. The sum of their ages 6 years ago was 42. What is the ratio of the current ages of the mother and the daughter?

(A) 18:7 (B) 7:5 (C) 6:1 (D) 13:5

D

One approach is to test the answer choices, and see which one works.

Here's the algebraic approach:

Let M = mother's present age
Let D = daughter's present age

6 years in the future
M + 6 = mother's age

Mother's age after 6 years will be thrice the age of her daughter now.
Presumably, this means that, in 6 years, the mom's age will be 3 times the daughter's current age.
So, (mother's age in 6 years) = 3(daughter's present age)
M + 6 = 3D
Simplify: 3D - M = 6

6 years ago
M - 6 = mother's age
D - 6 = daughter's age
The sum of their ages 6 years ago was 42.
So, (M - 6) + (D - 6) = 42
Simplify: D + M = 54

From here, if we solve this system . . .
3D - M = 6
D + M = 54

. . . we get D = 15 and M = 39,

What is the ratio of the current ages of the mother and the daughter?
mother's age : daughter's age = 39 : 15
= [spoiler]13 : 5[/spoiler]

Answer: D

Aside: If anyone is interested, we have a free video on solving GMAT age problems: https://www.gmatprepnow.com/module/gmat- ... ems?id=908
Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Mon Oct 21, 2013 2:51 pm

AIM TO CRACK GMAT wrote:A Mother's age after 6 years will be thrice the age of her daughter now. The sum of their ages 6 years ago was 42. What is the ratio of the current ages of the mother and the daughter?

(A) 18:7 (B) 7:5 (C) 6:1 (D) 13:5

6 years ago, the sum of the ages = 42.
Since mother and daughter are each now 6 years OLDER, the sum of their CURRENT ages = 42+6+6 = 54.
The answer choices represent the ratio of their current ages.
Since the sum of their current ages = 54, the sum of the parts of the correct ratio must be a FACTOR OF 54.
Only D works:
13+5 = 18, which is a factor of 54.

The correct answer is D.

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