The figure shows a tank made up of 3 sections. Cuboid X with a square base of side 9 cm and height 50 cm. Cube Y with a side of 12 cm. Cuboid Z that is 79 cm by 16 cm by 41 cm. Water from a tap above flows down at a rate of 2.7 litres per minute while water is drained out from the bottom at a rate of 1 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 Y
= 12 x 12 x 12
= 1728 cm³ 

Volume of Z
= 79 x 16 x 41
= 51824 cm³ 

1 ℓ = 1000 cm³ 
2.7 ℓ = 2700 cm³ 

Net increase in volume per minute
= 2700 - 1000
= 1700 cm³/min

Height of X to be filled with water
= 69 - (41 + 12)
= 16 cm

Volume of water in X
= 9 x 9 x 16
= 1296 cm³ 

Volume of water in Z, Y and part of X
= 51824 + 1728 + 1296
= 54848 cm³ 

Time taken to reach height
= 54848 ÷ 1700
≈ 32 min (Correct to nearest whole number)

Answer(s): 32 min