Distance/Time

Up stream

Distance - > 90
speed-> v-3
time-> t+1/2 = 2t+1/2

90 = 2t+1/2 (v-3)

180/(2t+1) - 3 = v (1)

Downstream
Distance - > 90
speed-> v+3
time-> t

90 = t(v+3)

90/t-3 = v (2)


Equate both v's from 1) and 2)


180/(2t+1) - 3 = 90/t-3

4t^2+2t-30 = 0

t=2.5

[90/(v-3)-90/(v+3)] = 1/2 hr

V=33

tdown = 90/36 = 2.5 hrs

Here is a link in GMAT Resources page which explains how to handle
such kind of problems.

https://www.beatthegmat.com/two-methods- ... t6749.html

HT Helps

cramya wrote:Up stream

Distance - > 90
speed-> v-3
time-> t+1/2 = 2t+1/2

90 = 2t+1/2 (v-3)

180/(2t+1) - 3 = v (1)

Downstream
Distance - > 90
speed-> v+3
time-> t

90 = t(v+3)

90/t-3 = v (2)


Equate both v's from 1) and 2)


180/(2t+1) - 3 = 90/t-3

4t^2+2t-30 = 0

t=2.5

cramya, something wrong with your calculation, I'm confused ..

let me do it step by step..

up : v-3= 90/(t+1/2)
v = 90/(t+1/2) +3 (not -3)
v = 180/(t+1) +3 ------(1)

down: v+3= 90/t
v= 90/t -3 ----------(2)

180/(t+1)+3 = 90/t-3
180/(t+1)+9 = 90/t
180t/(t+1)+ 9t = 90
180t + 18t^2+9t =180t+90
18t^2+9t-90 = 0
2t^2+t-10=0 until here and I stopped.. could you tell me what is wrong??