An empty rectangular tank has a capacity of 171 ℓ and a height of 100 cm. At 8 a.m., Tap A is turned on to fill the tank at a rate of 9 ℓ per minute for 15 minutes. After 7 minutes, Tap B is turned on to drain water from the tank at a rate of 9 ℓ per minute for 9 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 
9 ℓ = 9000 cm³ 
171 ℓ = 171000 cm³

Volume of water flowed in from Tap A
= 15 x 9000
= 135000 cm³

Volume of water from drained from Tap B
= 9 x 9000
= 81000 cm³

Volume of water left in the tank
= 135000 - 81000
= 54000 cm³

Base area of the tank
= 171000 ÷ 100
= 1710 cm² 

Height of water left in the tank
= 54000 ÷ 1710
≈ 31.6 cm

Answer(s): 31.6 cm