A tank holds 720 litres of water when full. An inlet pipe lets water in at 15 litres per minute, and an outlet pipe lets water out at 9 litres per minute. The tank is empty and both pipes are opened at the same time. How long will it take to fill the tank?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2 hours
With both pipes open, the tank gains the inflow minus the outflow every minute.
Net gain \(=15-9=6\) litres per minute
Time \(\displaystyle =\frac{720}{6}=120\) minutes
\(120\) minutes \(=2\) hours
The tank is full in 2 hours.