A rectangular container measuring 74 cm by 54 cm by 30 cm was to be filled with water by two taps, L and M. Tap L which fills the tank at a rate of 6 ℓ per minute was first turned on for 3 minutes before Tap M was turned on as well. If the tank was filled to the brim in a total of 7 minutes from the time when Tap L was first turned on, what is the rate at which Tap M fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the container
= 74 x 54 x 30
= 119880 cm³ 

1 ℓ = 1000 cm³ 
6 ℓ = 6000 cm³ 

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

Volume of water filled by Tap M
= 119880 - 42000
= 77880 cm³ 

Rate in which Tap M fills the container
= 77880 ÷ 4
19470 mℓ/min
= 19.47 ℓ/min
≈ 19.5 ℓ/min (Correct to 1 decimal place)

Answer(s): 19.5 ℓ/min