Gmat Prep RTD 2 min challenge RTD question

[khurram]

Ans is 90/(v-3)-90/(v+3) to solve to get answer

but it requires finding square root of 1071 I think. How am I supposed to fig out square in such a time of such a number.

Thanks
Khurram

[gmatinjuly]

I think you did a calculation mistake somewhere

90/v-3 - 90/ v+3 = 1/2

90 [ 1/v-3 - 1/v+3 ] = 1/2
1/v-3 - 1/v+3 = 1/180

v+3-v+3 / v^2 -9 = 1/180
6/(v^2 -9) = 1/180
v^2 - 9 = 180*6
v^2-9 = 1080
v^2 = 1089 (you subtracted 9 hence u got 1071)

1089 = 121 * 9

v = 11 * 3 = 33

time = 90/36 = 15 / 6 = 5 /2 = 2.5 hours

[khurram]

Is there a shortcut for this
1089 = 121 * 9

Hard to remember top of my head that 121*9-1089.

Thanks
khurram

[mim3]

Why not just plug in the answers and see if it computes? I did it that way and got it very quickly (I started with a). Even if you started with e and went backwards, it's at most a 2 minute problem.

[khurram]

how would u set up the equation. to test each answer

Thanks
Khurram