The figure, not drawn to scale, shows an empty container measuring 20 cm by 15 cm by 20 cm. Water flows from Tap G at a rate of 400 mℓ per minute and from Tap H at a rate of 230 mℓ per minute. Tower J and Tower K have a base area of 30 cm² and 30 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 15 minutes.
Volume of the container
= 20 x 15 x 20
= 6000 cm³ 

Volume of water flow per minute
= 400 + 230 - 130
= 500 cm³/min

Volume of water flow in 15 minutes
= 15 x 500
= 7500 cm³ 

Volume of water in the 2 towers
= 7500 - 6000
= 1500 cm³ 

Total combined base areas of 2 towers
= 30 + 30
= 60 cm² 

Height of water in 2 towers
= 1500 ÷ 60
= 25 cm

Total height of water
= 25 + 20
= 45 cm

Answer(s): 45 cm