The figure shows a tank made up of 3 sections. Cuboid N with a square base of side 9 cm and height 37 cm. Cube P with a side of 14 cm. Cuboid Q that is 80 cm by 11 cm by 46 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 tank to a height of 69 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of P
= 14 x 14 x 14
= 2744 cm³ 

Volume of Q
= 80 x 11 x 46
= 40480 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
= 69 - (46 + 14)
= 9 cm

Volume of water in N
= 9 x 9 x 9
= 729 cm³ 

Volume of water in Q, P and part of N
= 40480 + 2744 + 729
= 43953 cm³ 

Time taken to reach height
= 43953 ÷ 2100
≈ 21 min (Correct to nearest whole number)

Answer(s): 21 min