The figure shows a tank made up of 3 sections. Cuboid W with a square base of side 25 cm and height 31 cm. Cube X with a side of 10 cm. Cuboid Y that is 61 cm by 10 cm by 46 cm. Water from a tap above flows down at a rate of 3.9 litres per minute while water is drained out from the bottom at a rate of 2 litre per minute. How long did it take to fill up the tank to a height of 69 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of X
= 10 x 10 x 10
= 1000 cm³ 

Volume of Y
= 61 x 10 x 46
= 28060 cm³ 

2 ℓ = 2000 cm³ 
3.9 ℓ = 3900 cm³ 

Net increase in volume per minute
= 3900 - 2000
= 1900 cm³/min

Height of W to be filled with water
= 69 - (46 + 10)
= 13 cm

Volume of water in W
= 25 x 25 x 13
= 8125 cm³ 

Volume of water in Y, X and part of W
= 28060 + 1000 + 8125
= 37185 cm³ 

Time taken to reach height
= 37185 ÷ 1900
≈ 20 min (Correct to nearest whole number)

Answer(s): 20 min