Hectoliters of water

The pool has a total of 126 hectoliters of water. The first pump draws 2.1 liters of water per second. A second pump pumps 3.5 liters of water per second.

How long will it take both pumps to drain four-fifths of the water simultaneously?

Correct answer:

t =  1800 s

Step-by-step explanation:

$$\begin{gathered} V=126\text{hl}\to \text{l}=126\cdot 100\text{l}=12600\text{l} \\ Q=2.1+3.5=5.6\text{l/s} \\ {V}_2=\frac{4}{5}\cdot V=\frac{4}{5}\cdot 12600=\frac{4\cdot 12600}{5}=\frac{50400}{5}=10080\text{l} \\ t=V_2/Q=10080/\frac{28}{5}=10080:\frac{28}{5}=10080\cdot \frac{5}{28}=\frac{10080\cdot 5}{28}=\frac{50400}{28}=1800\text{s} \end{gathered}$$

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