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1. 3 pumps , working 8 hours a day, can empty a tank in 2 days. How many hours a day must 4 pumps work to empty the tank in 1 day ?

A. 9

B. 10

C. 11

D. 12

Solution:

Let the required number of working hours per day be x
More pumps, Less working hours per day (Indirect proportion)
Less days, More working hours per day (Indirect proportion)

\begin{matrix} Pumps  4 : 3 \\ Days 1 : 2 \end{matrix}} :: 8 : x

\begin{aligned} ∴ 4 \times 1 \times x = 3 \times 2 \times 8 \\ \Leftrightarrow x = \frac{(3 \times 2 \times 8)}{(4)} \\ \Leftrightarrow x = 12 \end{aligned}

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