A rectangular tank measuring 78 cm by 52 cm by 30 cm was to be filled with water by two taps, P and Q. Tap P which fills the tank at a rate of 9 ℓ per minute was first turned on for 3 minutes before Tap Q was turned on as well. If the tank was filled to the brim in a total of 7 minutes from the time when Tap P was first turned on, what is the rate at which Tap Q fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the tank
= 78 x 52 x 30
= 121680 cm
3 1 ℓ = 1000 cm
3 9 ℓ = 9000 cm
3 Volume of water filled by Tap P
= 9000 x 7
= 63000 cm
3 Volume of water filled by Tap Q
= 121680 - 63000
= 58680 cm
3 Rate in which Tap Q fills the tank
= 58680 ÷ 4
= 14670 cm3/min
14670 mℓ/min
= 14.67 ℓ/min
≈ 14.7 ℓ/min (Correct to 1 decimal place)
Answer(s): 14.7 ℓ/min