The figure shows a tank made up of 3 sections. Cuboid A with a square base of side 9 cm and height 45 cm. Cube B with a side of 17 cm. Cuboid C that is 80 cm by 20 cm by 47 cm. Water from a tap above flows down at a rate of 2.9 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 70 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of B
= 17 x 17 x 17
= 4913 cm³ 

Volume of C
= 80 x 20 x 47
= 75200 cm³ 

2 ℓ = 2000 cm³ 
2.9 ℓ = 2900 cm³ 

Net increase in volume per minute
= 2900 - 2000
= 900 cm³/min

Height of A to be filled with water
= 70 - (47 + 17)
= 6 cm

Volume of water in A
= 9 x 9 x 6
= 486 cm³ 

Volume of water in C, B and part of A
= 75200 + 4913 + 486
= 80599 cm³ 

Time taken to reach height
= 80599 ÷ 900
≈ 90 min (Correct to nearest whole number)

Answer(s): 90 min