a car was driving at 60 km / h for 20 minutes , and then at 90 km / h for another 40 minutes . what was its average speed ?
"driving at 60 km / h for 20 minutes , distance covered = 60 * 1 / 3 = 20 km driving at 90 km / h for 40 minutes , distance covered = 90 * 2 / 3 = 60 km average speed = total distance / total time = 80 / 1 = 80 km / h answer : a"
divide(add(multiply(divide(20, add(20, 40)), 60), multiply(divide(40, add(20, 40)), 90)), divide(add(20, 40), const_60))