The figure, not drawn to scale, shows an empty container measuring 20 cm by 15 cm by 15 cm. Water flows from Tap J at a rate of 500 mℓ per minute and from Tap K at a rate of 240 mℓ per minute. Tower L and Tower M have a base area of 20 cm² and 20 cm² respectively. Water is drained out of the container at a rate of 160 mℓ per minute. Find the height of the water level after 9 minutes.
Volume of the container
= 20 x 15 x 15
= 4500 cm³ 

Volume of water flow per minute
= 500 + 240 - 160
= 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
= 20 + 20
= 40 cm² 

Height of water in 2 towers
= 720 ÷ 40
= 18 cm

Total height of water
= 18 + 15
= 33 cm

Answer(s): 33 cm