Because of an anticipated rainstorm, the water level in a reservoir must be lowered by 1 foot. Opening spillway A lowers the level by this amount in 4 hours, whereas Opening spillway B does the job in 6 hours. How long will it take to lower the water level by 1 foot if both spillways are opened at the same time?
rate of spillway A = 1/4
rate of spillway B = 1/6
combined rate = 1/4 + 1/6 = 5/12
time at combined rate = 1/(5/12) = 12/5 or 2.4 hours