Question

A pipe fills a tank in 4 hours; another empties it in 8 hours. If both are opened, how long will it take to fill the tank?

• A. 4 hours
• B. 6 hours
• C. 8 hours
• D. 10 hours

Hint:

In pipes and cisterns, subtract rates if one pipe empties the tank.

Answer 1

The Correct Option is C

Concept: Net work rate = Filling rate $-$ Emptying rate. Explanation: Filling rate = $\frac{1}{4}$ tank/hour Emptying rate = $\frac{1}{8}$ tank/hour Net rate: \[ \frac{1}{4} - \frac{1}{8} = \frac{1}{8} \] So, time required = 8 hours.