Fire tank

The whole fire tank was discharged once in 4 days, first out by a second in $n day. Once firefighters pumped out 4/16 of water from the tank, they let the water flow out both drains.

How long does it take to empty the tank?

Correct answer:

t =  1.8 d

Step-by-step explanation:

$$\begin{gathered} t(\frac{1}{4}+\frac{1}{6})=1-\frac{4}{16} \\ t=\frac{1-\frac{4}{16}}{\frac{1}{4}+\frac{1}{6}} \\ t=1.8\text{d} \end{gathered}$$

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