The figure, not drawn to scale, shows an empty tank measuring 20 cm by 15 cm by 15 cm. Water flows from Tap P at a rate of 500 mℓ per minute and from Tap Q at a rate of 190 mℓ per minute. Tower R and Tower S have a base area of 50 cm² and 50 cm² respectively. Water is drained out of the container at a rate of 110 mℓ per minute. Find the height of the water level after 15 minutes.
Volume of the tank
= 20 x 15 x 15
= 4500 cm³ 

Volume of water flow per minute
= 500 + 190 - 110
= 580 cm³/min

Volume of water flow in 15 minutes
= 15 x 580
= 8700 cm³ 

Volume of water in the 2 towers
= 8700 - 4500
= 4200 cm³ 

Total combined base areas of 2 towers
= 50 + 50
= 100 cm² 

Height of water in 2 towers
= 4200 ÷ 100
= 42 cm

Total height of water
= 42 + 15
= 57 cm

Answer(s): 57 cm