Question

A boat travels 105 km downstream in 5 hours. The return trip upstream takes 7 hours. Find the speed of the boat in still water and the speed of the current.

Answer 1

speed of the boat = u
speed of the current = v

in downstream,
u - v = 105/5
u - v = 21

in upstream
u + v = 105/7
u + v = 15

add both equations
2u = 36
u = 18

v = -3

so
speed of the boat = 18 km/h
speed of the current = 3 km/h