GMAT Prep Test 1 PS 2

[ack]

Hi,

Does anyone know how to solve this problem?

Thanks

ack

[me4mba]

Upstream:
r = v-3
t = t+0.5
d = 90

Downstream:
r = v+3
t = t
d = 90

rt = d

RT(upstream) = RT(downstream) ; that is, 90 = 90

(v-3)(t+0.5) = (v+3)t

eventually, I got to 2t^2 + t - 15 = 0, so t = -3 or 2.5

The answer cannot be negative, so I chose 2.5

[neeraj_99]

instead of using algebra, better to backsolve

speed downstream is 6m/hr more than upstream speed, distance is same

A: t=2.5 hr, D=90, hence s=90/2.5=36m/hr
hence speed upstream=36-6=30 m/hr, hence t=90/30=3 hr,which satisfies the condition in the question