Three pumps

The water tank is emptied with the first pump in 6 hours and with the second pump in 2 hours. All three pumps had to be started at once to be emptied in 1 hour. How long will it take to empty the tank with only the third pump?

Correct answer:

c =  3 h

Step-by-step explanation:

$$\begin{gathered} 1\cdot (1/6+1/2+1/c)=1 \\ c/6+c/2+1=c \\ 0.333333c=1 \\ \frac{1}{3}c=1 \\ c=3=3\text{h} \end{gathered}$$

c/6 + c/2 + 1 = c

2c = 6


c = 6/2 = 3

c = 3

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