A rectangular tank measuring 86 cm by 57 cm by 22 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 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
= 86 x 57 x 22
= 107844 cm³ 

1 ℓ = 1000 cm³ 
7 ℓ = 7000 cm³ 

Volume of water filled by Tap P
= 7000 x 7
= 49000 cm³

Volume of water filled by Tap Q
= 107844 - 49000
= 58844 cm³ 

Rate in which Tap Q fills the tank
= 58844 ÷ 4
14711 mℓ/min
= 14.711 ℓ/min
≈ 14.7 ℓ/min (Correct to 1 decimal place)

Answer(s): 14.7 ℓ/min