A rectangular tank measuring 85 cm by 55 cm by 38 cm was to be filled with water by two taps, X and Y. Tap X which fills the tank at a rate of 7 ℓ per minute was first turned on for 4 minutes before Tap Y was turned on as well. If the tank was filled to the brim in a total of 6 minutes from the time when Tap X was first turned on, what is the rate at which Tap Y fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the tank
= 85 x 55 x 38
= 177650 cm
3 1 ℓ = 1000 cm
3 7 ℓ = 7000 cm
3 Volume of water filled by Tap X
= 7000 x 6
= 42000 cm
3 Volume of water filled by Tap Y
= 177650 - 42000
= 135650 cm
3 Rate in which Tap Y fills the tank
= 135650 ÷ 2
= 67825 cm3/min
67825 mℓ/min
= 67.825 ℓ/min
≈ 67.8 ℓ/min (Correct to 1 decimal place)
Answer(s): 67.8 ℓ/min