An empty rectangular tank has a capacity of 66 ℓ and a height of 10 cm. At 9 a.m., Tap R is turned on to fill the tank at a rate of 5 ℓ per minute for 20 minutes. After 9 minutes, Tap S is turned on to drain water from the tank at a rate of 4 ℓ per minute for 20 minutes. What is the height of the water level in the tank in the end? Give the answer in 1 decimal place.
1 ℓ = 1000 cm³ 
5 ℓ = 5000 cm³ 
4 ℓ = 4000 cm³ 
66 ℓ = 66000 cm³

Volume of water flowed in from Tap R
= 20 x 5000
= 100000 cm³

Volume of water from drained from Tap S
= 20 x 4000
= 80000 cm³

Volume of water left in the tank
= 100000 - 80000
= 20000 cm³

Base area of the tank
= 66000 ÷ 10
= 6600 cm² 

Height of water left in the tank
= 20000 ÷ 6600
≈ 3 cm

Answer(s): 3 cm