A rectangular container measuring 74 cm by 50 cm by 21 cm was to be filled with water by two taps, N and P. Tap N which fills the tank at a rate of 9 ℓ per minute was first turned on for 4 minutes before Tap P was turned on as well. If the tank was filled to the brim in a total of 6 minutes from the time when Tap N was first turned on, what is the rate at which Tap P fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the container
= 74 x 50 x 21
= 77700 cm³ 

1 ℓ = 1000 cm³ 
9 ℓ = 9000 cm³ 

Volume of water filled by Tap N
= 9000 x 6
= 54000 cm³

Volume of water filled by Tap P
= 77700 - 54000
= 23700 cm³ 

Rate in which Tap P fills the container
= 23700 ÷ 2
11850 mℓ/min
= 11.85 ℓ/min
≈ 11.9 ℓ/min (Correct to 1 decimal place)

Answer(s): 11.9 ℓ/min