A filling pipe can fill an empty tank in 30 minutes, and an emptying pipe can empty the full tank in 45 minutes. The tank is one-third full when both pipes are opened at the same time. In how many minutes will the tank be full?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 60 minutes
The tank already holds one-third, so the pipes only have to fill the remaining two-thirds at their net rate.
Net part filled in one minute \(\displaystyle =\frac{1}{30}-\frac{1}{45}=\frac{3-2}{90}=\frac{1}{90}\)
Part to be filled \(\displaystyle =1-\frac{1}{3}=\frac{2}{3}\)
Time \(\displaystyle =\frac{2}{3}\times90=60\) minutes