An empty rectangular tank has a capacity of 135 ℓ and a height of 15 cm. At 7 a.m., Tap Q is turned on to fill the tank at a rate of 9 ℓ per minute for 20 minutes. After 15 minutes, Tap R is turned on to drain water from the tank at a rate of 7 ℓ per minute for 25 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 
9 ℓ = 9000 cm³ 
7 ℓ = 7000 cm³ 
135 ℓ = 135000 cm³

Volume of water flowed in from Tap Q
= 20 x 9000
= 180000 cm³

Volume of water from drained from Tap R
= 25 x 7000
= 175000 cm³

Volume of water left in the tank
= 180000 - 175000
= 5000 cm³

Base area of the tank
= 135000 ÷ 15
= 9000 cm² 

Height of water left in the tank
= 5000 ÷ 9000
≈ 0.6 cm

Answer(s): 0.6 cm