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Age Problem -- using algebra

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Age Problem -- using algebra

by Stockmoose16 » Sat Oct 25, 2008 5:48 pm
Hello,

The following is from the OG quant book. I have tried solving it algebraically, but it never comes out correctly. Plugging in numbers works, but I want to know why Algebra isn't working.

Today Rose is twice as old as Sam and Sam is 3 years younger than Tina. If Rose, Sam, and Tina are all alive 4 years from today, which of the following must be true on that day.

I. Rose is twice as old as Sam
II. Sam is 3 years younger than Tina
III. Rose is older than Tina

(A) I only
(B) II only
(C) III only
(D) I and II
(E) II and III

OA is B

Here's my algebra:

R=2S
S=T-3

In 4 years:

R+4=2(S+4)
R+4=2S+8
R=2S+4

So (I) is correct by this algebra, but the correct answer says (I) doesn't work. What's wrong with my algebra here?
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by dadagreat » Sat Oct 25, 2008 6:12 pm
the assumption you made in the following equation is not right R+4=2(S+4). plug some numbers and you will realise this.

however - (S+4)=(T+4) - 3 is correct and will work out to the orig eq.
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by earth@work » Sat Oct 25, 2008 6:23 pm
Sam's age =S
Rose age = 2S
Tina age =S+3
4years later

sam =S+4 & Rose = 2S+4 .....so statement 1 is incorrect. U have taken rose's age 4 years later as 2(s+4) that is incorrect.
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