Question 71

A cistern has two taps whichfill it in 12 minutes and 15 minutes respectively. There is also
a waste pipe in the cistern. Whenall the three are opened, the empty cistern is full in 20
minutes. How long will the waste pipe take to empty the full cistern?

Solution

Let time taken by waste pipe to empty the cistern be $$t$$ minutes

=> $$\frac{1}{12}+\frac{1}{15}-\frac{1}{t}=\frac{1}{20}$$

=> $$\frac{1}{t}=\frac{1}{12}+\frac{1}{15}-\frac{1}{20}$$

=> $$\frac{1}{t}=\frac{5+4-3}{60}$$

=> $$\frac{1}{t}=\frac{6}{60}=\frac{1}{10}$$

$$\therefore$$ The waste pipe takes 10 min to empty the cistern.

=> Ans - (B)

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A cistern has two taps whichfill it in 12 minutes and 15 minutes respectively. There is alsoa waste pipe in the cistern. Whenall the three are opened, the empty cistern is full in 20minutes. How long will the waste pipe take to empty the full cistern?