Question

A large tanker can be filled by two pipes A and B in 60 minutes and 40 minutes respectively. How many minutes will it take to fill the tanker from an empty state, if B is used for half the time and A and B fill it together for the other half?

• A. 15 min
• B. 20 min
• C. 27.5 min
• D. 30 min

Answer 1

The correct option is D

30 min

Part filled by (A + B) in 1 minute $=(\frac{1}{60}+\frac{1}{40})=\frac{1}{24}$

Suppose the tank is filled in x minutes.

Then, $\frac{x}{2}(\frac{1}{24}+\frac{1}{40})=1\iff \frac{x}{2}\times \frac{1}{15}=1\iff x=30min.$