a man swims downstream 70 km and upstream 30 km taking 10 hours each time ; what is the speed of the current ?
"70 - - - 10 ds = 7 ? - - - - 1 30 - - - - 10 us = 3 ? - - - - 1 s = ? s = ( 7 - 3 ) / 2 = 2 answer : c"
divide(add(divide(30, 10), divide(70, 10)), const_2)