The figure shows a container made up of 3 sections. Cuboid N with a square base of side 25 cm and height 38 cm. Cube P with a side of 17 cm. Cuboid Q that is 72 cm by 10 cm by 41 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 1 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
= 17 x 17 x 17
= 4913 cm³ 

Volume of Q
= 72 x 10 x 41
= 29520 cm³ 

1 ℓ = 1000 cm³ 
3.1 ℓ = 3100 cm³ 

Net increase in volume per minute
= 3100 - 1000
= 2100 cm³/min

Height of N to be filled with water
= 68 - (41 + 17)
= 10 cm

Volume of water in N
= 25 x 25 x 10
= 6250 cm³ 

Volume of water in Q, P and part of N
= 29520 + 4913 + 6250
= 40683 cm³ 

Time taken to reach height
= 40683 ÷ 2100
≈ 19 min (Correct to nearest whole number)

Answer(s): 19 min