The figure shows a tank made up of 3 sections. Cuboid W with a square base of side 4 cm and height 48 cm. Cube X with a side of 15 cm. Cuboid Y that is 73 cm by 24 cm by 40 cm. Water from a tap above flows down at a rate of 3.4 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 61 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of X
= 15 x 15 x 15
= 3375 cm³ 

Volume of Y
= 73 x 24 x 40
= 70080 cm³ 

1 ℓ = 1000 cm³ 
3.4 ℓ = 3400 cm³ 

Net increase in volume per minute
= 3400 - 1000
= 2400 cm³/min

Height of W to be filled with water
= 61 - (40 + 15)
= 6 cm

Volume of water in W
= 4 x 4 x 6
= 96 cm³ 

Volume of water in Y, X and part of W
= 70080 + 3375 + 96
= 73551 cm³ 

Time taken to reach height
= 73551 ÷ 2400
≈ 31 min (Correct to nearest whole number)

Answer(s): 31 min