Question

A tank has an inlet pipe and an outlet pipe. If the outlet pipe is closed then the inlet pipe fills the empty tank in 8 hours. If the outlet pipe is open then the inlet pipe fills the empty tank in 10 hours. If only outlet pipe is open then in how many hours the full tank becomes half-full?

• A. 20
• B. 30
• C. 40
• D. 45

Answer 1

The correct option is A

20

Let the capacity of the tank be 100 L.
Then, efficiency of inlet pipe (outlet pipe is closed) = $\frac{100}{8}$ = 12.5 lph
Effective efficiency (when outlet pipe is open) = $\frac{100}{10}$ = 10 lph
Efficiency of outlet pipe = 12.5 – 10 – 2.5 lph.
Required time = $\frac{50}{2.5}$ = 20 hours.