Question

jane took 20min to drive her boat upstream to waterski at her favorite spot. Coming back later in the day, at the same boat speed, took her 15min. if the current in that part of the river is 3km per hr, what was the boat speed in still water?

Answer 1

Let boat speed in still water = xkm/h

given current speed in river = 3km/h

so,upstram speed = x-3km/h

so downstram speed=   x+3 km/h

and take total distance at one side = d km

given ,time upstream = 20min = 20/60hour

given, time downstram = 15 min = 15/60hour  

so, first eqn is

so, d/(x-3) = 20/60

so, 3d = x-3

so, d = (x-3)/3

& second  eqn is

so, d/(x+3 ) = 15/60

so, 4d = x+3

so, d = (x+3)/4

then from upper first eqn take d value & put here

so, (x-3)/3 = (x+3)/4

so, 4(x-3) = 3(x+3)

so, 4x-12 = 3x+9

so , 4x-3x = 12+9

so, x = 21

so, FINALLY speed of boat in still water = 21 km/h ANS