The figure shows a tank made up of 3 sections. Cuboid M with a square base of side 4 cm and height 45 cm. Cube N with a side of 17 cm. Cuboid P that is 71 cm by 23 cm by 44 cm. Water from a tap above flows down at a rate of 3.2 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 tank to a height of 68 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of N
= 17 x 17 x 17
= 4913 cm³ 

Volume of P
= 71 x 23 x 44
= 71852 cm³ 

2 ℓ = 2000 cm³ 
3.2 ℓ = 3200 cm³ 

Net increase in volume per minute
= 3200 - 2000
= 1200 cm³/min

Height of M to be filled with water
= 68 - (44 + 17)
= 7 cm

Volume of water in M
= 4 x 4 x 7
= 112 cm³ 

Volume of water in P, N and part of M
= 71852 + 4913 + 112
= 76877 cm³ 

Time taken to reach height
= 76877 ÷ 1200
≈ 64 min (Correct to nearest whole number)

Answer(s): 64 min