A rectangular container measuring 90 cm by 58 cm by 23 cm was to be filled with water by two taps, P and Q. Tap P which fills the tank at a rate of 7 ℓ per minute was first turned on for 2 minutes before Tap Q was turned on as well. If the tank was filled to the brim in a total of 6 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 container
= 90 x 58 x 23
= 120060 cm³ 

1 ℓ = 1000 cm³ 
7 ℓ = 7000 cm³ 

Volume of water filled by Tap P
= 7000 x 6
= 42000 cm³

Volume of water filled by Tap Q
= 120060 - 42000
= 78060 cm³ 

Rate in which Tap Q fills the container
= 78060 ÷ 4
19515 mℓ/min
= 19.515 ℓ/min
≈ 19.5 ℓ/min (Correct to 1 decimal place)

Answer(s): 19.5 ℓ/min