a car was driving at 72 km / h for 20 minutes , and then at 81 km / h for another 40 minutes . what was its average speed ?
driving at 72 km / h for 20 minutes , distance covered = 72 * 1 / 3 = 24 km driving at 90 km / h for 40 minutes , distance covered = 81 * 2 / 3 = 54 km average speed = total distance / total time = 78 / 1 = 78 km / h answer : a
divide(add(multiply(divide(20, add(20, 40)), 72), multiply(divide(40, add(20, 40)), 81)), divide(add(20, 40), const_60))