Fire tank

The whole fire tank was discharged once in 6 days, first out by a second in $n day. Once firefighters pumped out 4/5 of water from the tank, they let the water flow out both drains.

How long does it take to empty the tank?

Final Answer:

t =  0.4 d

Step-by-step explanation:

$$\begin{gathered} n=1-\frac{4}{5}=\frac{1\cdot 5}{5}-\frac{4}{5}=\frac{5}{5}-\frac{4}{5}=\frac{5-4}{5}=\frac{1}{5}=0.2 \\ {T}_1=1/6=\frac{1}{6}\doteq 0.16671\text{/h} \\ {T}_2=1/3=\frac{1}{3}\doteq 0.33331\text{/h} \\ t\cdot (T_1+T_2)=n \\ t=\frac{n}{T_1+T_2}=\frac{0.2}{0.1667+0.3333}=0.4\text{d} \end{gathered}$$

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