Three water pumps, A, B and C, can be used to fill a pool.
Pump A can fill the pool in two hours, pump B takes three hours, and pump C takes six hours.
How long does it take for the pool to fill if you use all three pumps at the same time?
The three pumps together fill the pool in exactly one hour.
A manages to fill half of the pool in one hour.
B fills a third of the pool in one hour.
C fills one sixth of the pool in one hour.
Therefore A, B and C complete the filling in exactly one hour.
1⁄2 + 1⁄3 + 1⁄6 = 1
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This puzzle originally appeared in Spektrum der Wissenschaft and was reproduced with permission.