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Re: What is the value of y^3 - t^3? [#permalink]
13 Sep 2012, 11:55

metallicafan wrote:

A fast way to solve this question? I solved it, but it took me more than 2 min :s.

What is the value of y^3 - t^3? (1) y - t = 3 (2) y^2 - t^2 = 21

I spent a lot of time in solving the equations with the possible values of y and t.

Source: gmathacks.com

(1) - (y-t) = 3 ----> Y & t can be anything -Insufficient (2) (y+t)(y-t) = 3 x 7 ----> (y+t) can be 3 or 7 -----> i.e. y will be 5 for sure but t can be 2 or -2 ----> Insufficient (1+2) (y+t)= 7 because (y-t) = 3----> y = 5 & t=2--->Sufficient

Hope it helps.
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Re: What is the value of y^3 - t^3? [#permalink]
14 Sep 2012, 03:41

1

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Expert's post

What is the value of y^3 - t^3?

(1) y - t = 3. If y=3 and t=0, we have one value for y^3 - t^3 and for y=4 and t=1 another. Not sufficient.

(2) y^2 - t^2 = 21. The same here. Not sufficient.

(1)+(2) From (2) we have (y-t)(y+t)=21. Since from (1) y - t = 3, then 3(y+t)=21 --> y+t=7. So we have two distinct linear equation with two unknowns: y+t=7 and y - t = 3, which means that we can solve for them and get the value of y^3 - t^3. Sufficient.

Re: What is the value of y^3 - t^3? [#permalink]
10 Nov 2013, 19:29

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