A rectangular tank measuring 84 cm by 57 cm by 21 cm was to be filled with water by two taps, A and B. Tap A which fills the tank at a rate of 7 ℓ per minute was first turned on for 3 minutes before Tap B was turned on as well. If the tank was filled to the brim in a total of 5 minutes from the time when Tap A was first turned on, what is the rate at which Tap B fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the tank
= 84 x 57 x 21
= 100548 cm³ 

1 ℓ = 1000 cm³ 
7 ℓ = 7000 cm³ 

Volume of water filled by Tap A
= 7000 x 5
= 35000 cm³

Volume of water filled by Tap B
= 100548 - 35000
= 65548 cm³ 

Rate in which Tap B fills the tank
= 65548 ÷ 2
32774 mℓ/min
= 32.774 ℓ/min
≈ 32.8 ℓ/min (Correct to 1 decimal place)

Answer(s): 32.8 ℓ/min