A rectangular container measuring 71 cm by 50 cm by 21 cm was to be filled with water by two taps, C and D. Tap C which fills the tank at a rate of 9 ℓ per minute was first turned on for 3 minutes before Tap D was turned on as well. If the tank was filled to the brim in a total of 6 minutes from the time when Tap C was first turned on, what is the rate at which Tap D fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the container
= 71 x 50 x 21
= 74550 cm
3 1 ℓ = 1000 cm
3 9 ℓ = 9000 cm
3 Volume of water filled by Tap C
= 9000 x 6
= 54000 cm
3 Volume of water filled by Tap D
= 74550 - 54000
= 20550 cm
3 Rate in which Tap D fills the container
= 20550 ÷ 3
= 6850 cm3/min
6850 mℓ/min
= 6.85 ℓ/min
≈ 6.9 ℓ/min (Correct to 1 decimal place)
Answer(s): 6.9 ℓ/min