We're given the rates of two entities that will share a job together - this is the standard "set-up" for a Work Formula question.
Work Formula = (A)(B)/(A+B) where A and B are the respective amounts of time needed to individually complete a job.
Pump A = 2 hours to fill a tank
Pump B = 1/2 hour to fill a tank
Working together, it takes....
(2)(1/2)/(2 + 1/2) = 1/2.5 hours to fill the tank together...
1/2.5 = 2/5 hours
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