The figure shows a tank made up of 3 sections. Cuboid C with a square base of side 9 cm and height 34 cm. Cube D with a side of 11 cm. Cuboid E that is 73 cm by 13 cm by 42 cm. Water from a tap above flows down at a rate of 3.1 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 67 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of D
= 11 x 11 x 11
= 1331 cm³ 

Volume of E
= 73 x 13 x 42
= 39858 cm³ 

1 ℓ = 1000 cm³ 
3.1 ℓ = 3100 cm³ 

Net increase in volume per minute
= 3100 - 1000
= 2100 cm³/min

Height of C to be filled with water
= 67 - (42 + 11)
= 14 cm

Volume of water in C
= 9 x 9 x 14
= 1134 cm³ 

Volume of water in E, D and part of C
= 39858 + 1331 + 1134
= 42323 cm³ 

Time taken to reach height
= 42323 ÷ 2100
≈ 20 min (Correct to nearest whole number)

Answer(s): 20 min