Jack mows his lawn in four hours. Marylin can mow the same yard in six hours. How much time would it take for them to mow the yard together?

Jack will mow 1/4 per hour. Marilyn will mow 1/6 per hour.

1/4 + 1/6 = 3/12 + 2/12 = 5/12

Together they will mow 5/12 per hour.

Total hours x mowed per hour = 1

Hours x 5/12 = 1

Hours = 1/(5/12)

Hours = 2.4