Pond

A pond with two open floodgates will emptied in 20 days. A larger floodgate will emptied whole pond in 30 days. For how long would he be emptied with a smaller floodgate.

Result

t =  60 h

Solution:

$20 \cdot \ (1/t+1/30)=1 \ \\ 20+ 20/30t=t \ \\ t=20/(1-20/30)=60 \ \text{h}$

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