Pipe A can fill an empty tank in 24 minutes and pipe B can fill it in 36 minutes. The tank is empty. Pipe A is opened first, and 4 minutes later pipe B is also opened; both then stay open. How many minutes after pipe A was opened will the tank be full?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 16 minutes
Take the capacity as the LCM of 24 and 36, that is 72 units. A works alone for the first 4 minutes, and then both pipes fill the rest together.
A fills \(\frac{72}{24}=3\) units and B fills \(\frac{72}{36}=2\) units per minute
A alone in 4 minutes: \(4\times3=12\) units
Left to fill \(=72-12=60\) units
A and B together: \(\displaystyle \frac{60}{3+2}=12\) minutes
Total time from the start \(=4+12=16\) minutes