a boat goes 100 km downstream in 4 hours , and 75 km upstream in 15 hours . the speed of the stream is ?
"100 - - - 10 ds = 25 ? - - - - 1 75 - - - - 15 us = 5 ? - - - - - 1 s = ( 25 - 5 ) / 2 = 10 kmph . answer : a"
divide(subtract(divide(100, 4), divide(75, 15)), const_2)