If the painter works four times as fast as his assistant, how many hours will it take them both working together to paint a house that would have taken the painter 20 hours to complete by himself?

Question

ACT question that do not know how to solve

Answer 1

The painter can paint one house in 20 hours, so his rate of working is 1/20 house per hour. His assistant would take 4 times as long—80 hours.

His assistant’s rate is 1/80 house per hour.

Their combined rate is 1/20+1/80=4/80+1/80=5/80=1/16 house per hour.

1/16 house per hour = 16 hours for one house, so it takes them 16 hours working together.