Jack usually mows his lawn in 4 hours. Marilyn can mow the same yard in 5 hours. How much time would it take for them to mow the lawn together?

Answer 1

1/t= 1/4 + 1/5,
1/t = 5/20 + 4/20 = 9/20,
t = 20/9 = 2.2 hours.

Or
t = 4*5 / (4+5) = 20/9 = 2.2 hours.