A rectangular tank measuring 81 cm by 53 cm by 25 cm was to be filled with water by two taps, U and V. Tap U which fills the tank at a rate of 6 ℓ per minute was first turned on for 4 minutes before Tap V was turned on as well. If the tank was filled to the brim in a total of 7 minutes from the time when Tap U was first turned on, what is the rate at which Tap V fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the tank
= 81 x 53 x 25
= 107325 cm³ 

1 ℓ = 1000 cm³ 
6 ℓ = 6000 cm³ 

Volume of water filled by Tap U
= 6000 x 7
= 42000 cm³

Volume of water filled by Tap V
= 107325 - 42000
= 65325 cm³ 

Rate in which Tap V fills the tank
= 65325 ÷ 3
21775 mℓ/min
= 21.775 ℓ/min
≈ 21.8 ℓ/min (Correct to 1 decimal place)

Answer(s): 21.8 ℓ/min