3 pumps were used to empty a pond hold an unknown amount of water. If pump 1 can empty it in 4 days, pump 2 in 10 days and #3 in 5 days, how many days will it take to empty the pond if all 3 are running at the same time?

Answer 1

Let x = time (days) for all 3
then
x(1/4 + 1/10 + 1/5) = 1
multiplying both sides by 20
x(5 + 2 + 4) = 20
x(11) = 20
x = 20/11
x = 1.82 days
or
x = 1 day, 19 hours and 38 minutes