find avrg speed if a man travels at speed of 18 km / hr up and 24 km / hr dawn at an altitude of 300 m .
"avg speed = 2 * x * y / ( x + y ) = 2 * 18 * 24 / ( 18 + 24 ) = 20.5 answer : d"
divide(multiply(multiply(const_2, 18), 24), add(18, 24))