The figure shows a tank made up of 3 sections. Cuboid Q with a square base of side 4 cm and height 44 cm. Cube R with a side of 16 cm. Cuboid S that is 71 cm by 24 cm by 44 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 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 R
= 16 x 16 x 16
= 4096 cm³ 

Volume of S
= 71 x 24 x 44
= 74976 cm³ 

2 ℓ = 2000 cm³ 
3.1 ℓ = 3100 cm³ 

Net increase in volume per minute
= 3100 - 2000
= 1100 cm³/min

Height of Q to be filled with water
= 69 - (44 + 16)
= 9 cm

Volume of water in Q
= 4 x 4 x 9
= 144 cm³ 

Volume of water in S, R and part of Q
= 74976 + 4096 + 144
= 79216 cm³ 

Time taken to reach height
= 79216 ÷ 1100
≈ 72 min (Correct to nearest whole number)

Answer(s): 72 min