Mr. Fat needs 20 minutes to eat a pound of cereal, while Mr. Thin needs 30 minutes. If they eat together, how long does it take for them to finish off three pounds of cereal? Express your answer in minutes.
It would take 60 minutes for Mr. F. to eat 3 pounds of cereal, and it would take 90 minutes for Mr. T. To eat 3 pounds of cereal, so if they were eating together it would take 90 minutes for them to finish.
F rate 1 lb / 20 min
T rate 1 lb /30 min
3 / (1/20 + 1/30) = 36 minutes