The figure shows a container made up of 3 sections. Cuboid L with a square base of side 9 cm and height 24 cm. Cube M with a side of 16 cm. Cuboid N that is 72 cm by 11 cm by 40 cm. Water from a tap above flows down at a rate of 3.9 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 70 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of M
= 16 x 16 x 16
= 4096 cm³ 

Volume of N
= 72 x 11 x 40
= 31680 cm³ 

1 ℓ = 1000 cm³ 
3.9 ℓ = 3900 cm³ 

Net increase in volume per minute
= 3900 - 1000
= 2900 cm³/min

Height of L to be filled with water
= 70 - (40 + 16)
= 14 cm

Volume of water in L
= 9 x 9 x 14
= 1134 cm³ 

Volume of water in N, M and part of L
= 31680 + 4096 + 1134
= 36910 cm³ 

Time taken to reach height
= 36910 ÷ 2900
≈ 13 min (Correct to nearest whole number)

Answer(s): 13 min