The figure shows a tank made up of 3 sections. Cuboid L with a square base of side 4 cm and height 45 cm. Cube M with a side of 11 cm. Cuboid N that is 60 cm by 13 cm by 50 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 tank to a height of 70 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of M
= 11 x 11 x 11
= 1331 cm³ 

Volume of N
= 60 x 13 x 50
= 39000 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 - (50 + 11)
= 9 cm

Volume of water in L
= 4 x 4 x 9
= 144 cm³ 

Volume of water in N, M and part of L
= 39000 + 1331 + 144
= 40475 cm³ 

Time taken to reach height
= 40475 ÷ 2900
≈ 14 min (Correct to nearest whole number)

Answer(s): 14 min