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DS: age

by 7806 » Sat Sep 04, 2010 12:37 pm
Bobby and his younger brother Johnny have the same birthday. Johnny's age now is the same as Bobby's age was when Johnny was half as old as Bobby is now. What is Bobby's age now?
(1) Bobby is currently four times as old as he was when Johnny was born.
(2) Bobby was six years old when Johnny was born.

pl explain your approach. thanks.



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Source: — Data Sufficiency |

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by Rahul@gurome » Sat Sep 04, 2010 9:02 pm
Solution:
Let Johnny's present age be x and Bobby's present age be y.
Half of Bobby's age is y/2.
Johnny must have been y/2 years of age (x-y/2) years ago.
At that time Bobby must have been y - (x - y/2) = 3y/2 - x years of age.
So x = 3y/2 - x
Or 2x = 3y/2.
Or 4x = 3y.

Consider first statement (1) alone.
When Johnny was born, Bobby must have been y - x years of age.
Or 4*(y-x) = y
Or 3y = 4x.
This is just the same equation as what we get from the main question.
So (1) alone is not sufficient.

Next consider (2) alone.
It means y - x = 6.
So we have 2 equations, 4x = 3y and y - x = 6.
Or x = 18 and y is 24.

So Bobby's age now is 24 years.

Since (2) alone is sufficient, (B) is the correct answer.
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by 7806 » Sun Sep 05, 2010 11:48 am
thanks rahul...nice explanation.
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