a river boat leaves silver town and travels upstream to gold town at an average speed of 3 kilometers per hour . it returns by the same route at an average speed of 7 kilometers per hour . what is the average speed for the round - trip in kilometers per hour ?
pick a number which is lcm of 7 and 3 = 21 . upstream time = 21 / 3 = 7 hrs downstream time = 21 / 7 = 3 hrs total time = 10 hrs total distance = 42 average speed = 42 / 10 = 4.2 km / hr
divide(multiply(multiply(3, 7), const_2), add(3, 7))