A girl rows 10 km downstream in 2 h. Her return trip takes 4 h. How fast can she row in still water. in km/h?
rowing speed = v
speed upstream = (v-c)
speed downstream = (v+c)
(v+c) * 2 = 10
(v-c) * 4 = 10 or (v-c) * 2 = 5
2 v + 2 c = 10
2 v - 2 c = 5
------------------add
4 v = 15
v = 15/4 = 3.75 km/hr