The figure shows a tank made up of 3 sections. Cuboid Q with a square base of side 9 cm and height 39 cm. Cube R with a side of 20 cm. Cuboid S that is 76 cm by 28 cm by 41 cm. Water from a tap above flows down at a rate of 2.5 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 70 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of R
= 20 x 20 x 20
= 8000 cm³ 

Volume of S
= 76 x 28 x 41
= 87248 cm³ 

1 ℓ = 1000 cm³ 
2.5 ℓ = 2500 cm³ 

Net increase in volume per minute
= 2500 - 1000
= 1500 cm³/min

Height of Q to be filled with water
= 70 - (41 + 20)
= 9 cm

Volume of water in Q
= 9 x 9 x 9
= 729 cm³ 

Volume of water in S, R and part of Q
= 87248 + 8000 + 729
= 95977 cm³ 

Time taken to reach height
= 95977 ÷ 1500
≈ 64 min (Correct to nearest whole number)

Answer(s): 64 min