The figure shows a container made up of 3 sections. Cuboid N with a square base of side 4 cm and height 23 cm. Cube P with a side of 15 cm. Cuboid Q that is 77 cm by 25 cm by 47 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 container to a height of 68 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of P
= 15 x 15 x 15
= 3375 cm³ 

Volume of Q
= 77 x 25 x 47
= 90475 cm³ 

2 ℓ = 2000 cm³ 
3.1 ℓ = 3100 cm³ 

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

Height of N to be filled with water
= 68 - (47 + 15)
= 6 cm

Volume of water in N
= 4 x 4 x 6
= 96 cm³ 

Volume of water in Q, P and part of N
= 90475 + 3375 + 96
= 93946 cm³ 

Time taken to reach height
= 93946 ÷ 1100
≈ 85 min (Correct to nearest whole number)

Answer(s): 85 min