The figure, not drawn to scale, shows an empty container measuring 20 cm by 15 cm by 15 cm. Water flows from Tap D at a rate of 400 mℓ per minute and from Tap E at a rate of 210 mℓ per minute. Tower F and Tower G have a base area of 10 cm² and 20 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 11 minutes.
Volume of the container
= 20 x 15 x 15
= 4500 cm³ 

Volume of water flow per minute
= 400 + 210 - 130
= 480 cm³/min

Volume of water flow in 11 minutes
= 11 x 480
= 5280 cm³ 

Volume of water in the 2 towers
= 5280 - 4500
= 780 cm³ 

Total combined base areas of 2 towers
= 10 + 20
= 30 cm² 

Height of water in 2 towers
= 780 ÷ 30
= 26 cm

Total height of water
= 26 + 15
= 41 cm

Answer(s): 41 cm