The figure shows a tank made up of 3 sections. Cuboid F with a square base of side 4 cm and height 26 cm. Cube G with a side of 18 cm. Cuboid H that is 72 cm by 16 cm by 42 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 68 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of G
= 18 x 18 x 18
= 5832 cm³ 

Volume of H
= 72 x 16 x 42
= 48384 cm³ 

2 ℓ = 2000 cm³ 
2.9 ℓ = 2900 cm³ 

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

Height of F to be filled with water
= 68 - (42 + 18)
= 8 cm

Volume of water in F
= 4 x 4 x 8
= 128 cm³ 

Volume of water in H, G and part of F
= 48384 + 5832 + 128
= 54344 cm³ 

Time taken to reach height
= 54344 ÷ 900
≈ 60 min (Correct to nearest whole number)

Answer(s): 60 min