The figure shows a tank made up of 3 sections. Cuboid A with a square base of side 9 cm and height 36 cm. Cube B with a side of 13 cm. Cuboid C that is 79 cm by 30 cm by 46 cm. Water from a tap above flows down at a rate of 3.3 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 B
= 13 x 13 x 13
= 2197 cm³ 

Volume of C
= 79 x 30 x 46
= 109020 cm³ 

2 ℓ = 2000 cm³ 
3.3 ℓ = 3300 cm³ 

Net increase in volume per minute
= 3300 - 2000
= 1300 cm³/min

Height of A to be filled with water
= 69 - (46 + 13)
= 10 cm

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

Volume of water in C, B and part of A
= 109020 + 2197 + 810
= 112027 cm³ 

Time taken to reach height
= 112027 ÷ 1300
≈ 86 min (Correct to nearest whole number)

Answer(s): 86 min