A rectangular tank measuring 86 cm by 51 cm by 30 cm was to be filled with water by two taps, P and Q. Tap P which fills the tank at a rate of 7 ℓ per minute was first turned on for 3 minutes before Tap Q was turned on as well. If the tank was filled to the brim in a total of 7 minutes from the time when Tap P was first turned on, what is the rate at which Tap Q fills the tank? Give your answer in ℓ per minute correct to 1 decimal place.
Total volume of the tank
= 86 x 51 x 30
= 131580 cm
3 1 ℓ = 1000 cm
3 7 ℓ = 7000 cm
3 Volume of water filled by Tap P
= 7000 x 7
= 49000 cm
3 Volume of water filled by Tap Q
= 131580 - 49000
= 82580 cm
3 Rate in which Tap Q fills the tank
= 82580 ÷ 4
= 20645 cm3/min
20645 mℓ/min
= 20.645 ℓ/min
≈ 20.6 ℓ/min (Correct to 1 decimal place)
Answer(s): 20.6 ℓ/min