A rectangular container measuring 81 cm by 51 cm by 21 cm was to be filled with water by two taps, M and N. Tap M which fills the tank at a rate of 7 ℓ per minute was first turned on for 4 minutes before Tap N was turned on as well. If the tank was filled to the brim in a total of 5 minutes from the time when Tap M was first turned on, what is the rate at which Tap N fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the container
= 81 x 51 x 21
= 86751 cm
3 1 ℓ = 1000 cm
3 7 ℓ = 7000 cm
3 Volume of water filled by Tap M
= 7000 x 5
= 35000 cm
3 Volume of water filled by Tap N
= 86751 - 35000
= 51751 cm
3 Rate in which Tap N fills the container
= 51751 ÷ 1
= 51751 cm3/min
51751 mℓ/min
= 51.751 ℓ/min
≈ 51.8 ℓ/min (Correct to 1 decimal place)
Answer(s): 51.8 ℓ/min