Question 598485
There are two small valves in the bottom of a tub filled with water. If opened, one valve will empty the tub in 3 hours; the other will empty it in 6 hours. If both valves are opened at the same time, how long will it take to empty the tub? Please show work. 
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Let t = time (hours) to empty tub by both valves
then
t(1/3 + 1/6) = 1
multiplying both sides by 6, we get:
t(2 + 1) = 6
t(3) = 6
t = 6/3
t = 2 hours