A rectangular tank measuring 75 cm by 50 cm by 23 cm was to be filled with water by two taps, X and Y. Tap X which fills the tank at a rate of 9 ℓ per minute was first turned on for 4 minutes before Tap Y was turned on as well. If the tank was filled to the brim in a total of 7 minutes from the time when Tap X was first turned on, what is the rate at which Tap Y fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the tank
= 75 x 50 x 23
= 86250 cm³ 

1 ℓ = 1000 cm³ 
9 ℓ = 9000 cm³ 

Volume of water filled by Tap X
= 9000 x 7
= 63000 cm³

Volume of water filled by Tap Y
= 86250 - 63000
= 23250 cm³ 

Rate in which Tap Y fills the tank
= 23250 ÷ 3
7750 mℓ/min
= 7.75 ℓ/min
≈ 7.8 ℓ/min (Correct to 1 decimal place)

Answer(s): 7.8 ℓ/min