The figure shows a tank made up of 3 sections. Cuboid V with a square base of side 9 cm and height 47 cm. Cube W with a side of 13 cm. Cuboid X that is 75 cm by 21 cm by 45 cm. Water from a tap above flows down at a rate of 2.6 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 68 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of W
= 13 x 13 x 13
= 2197 cm³ 

Volume of X
= 75 x 21 x 45
= 70875 cm³ 

1 ℓ = 1000 cm³ 
2.6 ℓ = 2600 cm³ 

Net increase in volume per minute
= 2600 - 1000
= 1600 cm³/min

Height of V to be filled with water
= 68 - (45 + 13)
= 10 cm

Volume of water in V
= 9 x 9 x 10
= 810 cm³ 

Volume of water in X, W and part of V
= 70875 + 2197 + 810
= 73882 cm³ 

Time taken to reach height
= 73882 ÷ 1600
≈ 46 min (Correct to nearest whole number)

Answer(s): 46 min