Pipe A can fill an empty tank in 20 minutes and pipe B can fill it in 30 minutes. The tank is empty. Pipe A is opened first, and 5 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 3
The correct answer is Option (3) → 14 minutes
Take the capacity as the LCM of 20 and 30, that is 60 units. A works alone for the first 5 minutes, and then both pipes fill the rest together.
A fills \(\frac{60}{20}=3\) units and B fills \(\frac{60}{30}=2\) units per minute
A alone in 5 minutes: \(5\times3=15\) units
Left to fill \(=60-15=45\) units
A and B together: \(\displaystyle \frac{45}{3+2}=9\) minutes
Total time from the start \(=5+9=14\) minutes