The figure shows a container made up of 3 sections. Cuboid N with a square base of side 9 cm and height 27 cm. Cube P with a side of 11 cm. Cuboid Q that is 70 cm by 12 cm by 50 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 67 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of P
= 11 x 11 x 11
= 1331 cm³ 

Volume of Q
= 70 x 12 x 50
= 42000 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
= 67 - (50 + 11)
= 6 cm

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

Volume of water in Q, P and part of N
= 42000 + 1331 + 486
= 43817 cm³ 

Time taken to reach height
= 43817 ÷ 1100
≈ 40 min (Correct to nearest whole number)

Answer(s): 40 min