There are two small valves in the bottom of a tub filled with water. If opened, one valve will empty the tub in three hours; the other will empty it in six hours. If both valves are opened at the same time, how long will it take to empty the tub?

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The small one empties 1/6 of the tub per hour.

The other one 1/3 per hour.

Add them

1/6 + 1/3 = 3/6 = 1/2 tub per hour.

--> 2 hours.

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A shortcut: product/sum

3*6/(3+6) = 18/9

= 2 hours