find avrg speed if a man travels at speed of 15 km / hr up and 28 km / hr dawn at an altitude of 230 m .
"avg speed = 2 * x * y / ( x + y ) = 2 * 15 * 28 / ( 15 + 28 ) = 19.5 answer : b"
divide(multiply(multiply(const_2, 15), 28), add(15, 28))