Gprep PS

Speed = distance/time

use this and set up the 2 equations

For going up the river :
90 = t * (v - 3)

For going down the river :
90 = (t - 0.5) * (v + 3)

solve for the equations.to get = 2.5

hence 2.5 for downstream and 3 hrs for upstream..

hope it helps..

PS: be careful while solving the equation...

Thanks , the answer is 2.5.

Hi Sudhir ,

I somehow feel/I coudnt solve the equations ! Could u please complete the solution??

Vishu

vishubn wrote:Hi Sudhir ,

I somehow feel/I coudnt solve the equations ! Could u please complete the solution??

Vishu

Distance = Rate * Time, so...

90 = (v-3) * (t+1/2) - since it took half an hour longer to go upstream, and 90 = (v+3) * t.

Expand these out:

90 = vt - 3t + v/2 - 3/2...........................................1
90 = vt + 3t............................................................2

Subtract the first from the second:

0 = 6t - v/2 + 3/2

Solve for t in terms of v:

6t = v/2 - 3/2
t = v/12 - 1/4

Substitute into the second equation so we can solve for t:

90 = vt+3t = v(v/12-1/4)+3(v/12-1/4)
= v^2/12-v/4+v/4-3/4
= v^2/12 - 3/4

v = sqrt((90+3/4)*12)
= sqrt(363/4*12)
= sqrt(1089) = 33

And since t = v/12-1/4:
t = 33/12-1/4 = 33/12-3/12 = 30/12 = 5/2. It took 2 1/2 hours downstream and 3 hours upstream


This would probably take a lot of time ...

i just realised there a better way of doing it as well.....


try this

T2-T1 = 1/2
So (90/v-3) - (90/v+3) = 1/2
1089 = v^2
v = 33

so t=2.5

Thanks Sudhir

Yes indeed i went to on to solve the equation after i posted the req ! But Yes second approach was coool

Vishu

Time taken upstream = 90/v-3
Time taken downstream = 90/v+3

Hence

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

v = 33

Time taken downstream = 90/36 = 5/2 = 2.5 hrs simple