The figure shows a tank made up of 3 sections. Cuboid U with a square base of side 14 cm and height 44 cm. Cube V with a side of 10 cm. Cuboid W that is 71 cm by 11 cm by 49 cm. Water from a tap above flows down at a rate of 3.5 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 67 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of V
= 10 x 10 x 10
= 1000 cm³ 

Volume of W
= 71 x 11 x 49
= 38269 cm³ 

1 ℓ = 1000 cm³ 
3.5 ℓ = 3500 cm³ 

Net increase in volume per minute
= 3500 - 1000
= 2500 cm³/min

Height of U to be filled with water
= 67 - (49 + 10)
= 8 cm

Volume of water in U
= 14 x 14 x 8
= 1568 cm³ 

Volume of water in W, V and part of U
= 38269 + 1000 + 1568
= 40837 cm³ 

Time taken to reach height
= 40837 ÷ 2500
≈ 16 min (Correct to nearest whole number)

Answer(s): 16 min