The figure, not drawn to scale, shows an empty tank measuring 20 cm by 15 cm by 15 cm. Water flows from Tap A at a rate of 500 mℓ per minute and from Tap B at a rate of 210 mℓ per minute. Tower C and Tower D have a base area of 40 cm² and 40 cm² respectively. Water is drained out of the container at a rate of 130 mℓ per minute. Find the height of the water level after 9 minutes.
Volume of the tank
= 20 x 15 x 15
= 4500 cm³ 

Volume of water flow per minute
= 500 + 210 - 130
= 580 cm³/min

Volume of water flow in 9 minutes
= 9 x 580
= 5220 cm³ 

Volume of water in the 2 towers
= 5220 - 4500
= 720 cm³ 

Total combined base areas of 2 towers
= 40 + 40
= 80 cm² 

Height of water in 2 towers
= 720 ÷ 80
= 9 cm

Total height of water
= 9 + 15
= 24 cm

Answer(s): 24 cm