The figure, not drawn to scale, shows an empty container measuring 20 cm by 15 cm by 15 cm. Water flows from Tap K at a rate of 300 mℓ per minute and from Tap L at a rate of 230 mℓ per minute. Tower M and Tower N have a base area of 20 cm² and 50 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 13 minutes.
Volume of the container
= 20 x 15 x 15
= 4500 cm³ 

Volume of water flow per minute
= 300 + 230 - 130
= 400 cm³/min

Volume of water flow in 13 minutes
= 13 x 400
= 5200 cm³ 

Volume of water in the 2 towers
= 5200 - 4500
= 700 cm³ 

Total combined base areas of 2 towers
= 20 + 50
= 70 cm² 

Height of water in 2 towers
= 700 ÷ 70
= 10 cm

Total height of water
= 10 + 15
= 25 cm

Answer(s): 25 cm