A rectangular tank measuring 85 cm by 50 cm by 21 cm was to be filled with water by two taps, Q and R. Tap Q which fills the tank at a rate of 7 ℓ per minute was first turned on for 3 minutes before Tap R was turned on as well. If the tank was filled to the brim in a total of 6 minutes from the time when Tap Q was first turned on, what is the rate at which Tap R fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the tank
= 85 x 50 x 21
= 89250 cm
3 1 ℓ = 1000 cm
3 7 ℓ = 7000 cm
3 Volume of water filled by Tap Q
= 7000 x 6
= 42000 cm
3 Volume of water filled by Tap R
= 89250 - 42000
= 47250 cm
3 Rate in which Tap R fills the tank
= 47250 ÷ 3
= 15750 cm3/min
15750 mℓ/min
= 15.75 ℓ/min
≈ 15.8 ℓ/min (Correct to 1 decimal place)
Answer(s): 15.8 ℓ/min