A rectangular tank measuring 80 cm by 60 cm by 36 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 5 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
= 80 x 60 x 36
= 172800 cm
3 1 ℓ = 1000 cm
3 7 ℓ = 7000 cm
3 Volume of water filled by Tap Q
= 7000 x 5
= 35000 cm
3 Volume of water filled by Tap R
= 172800 - 35000
= 137800 cm
3 Rate in which Tap R fills the tank
= 137800 ÷ 2
= 68900 cm3/min
68900 mℓ/min
= 68.9 ℓ/min
≈ 68.9 ℓ/min (Correct to 1 decimal place)
Answer(s): 68.9 ℓ/min