The figure shows a tank made up of 3 sections. Cuboid T with a square base of side 25 cm and height 49 cm. Cube U with a side of 12 cm. Cuboid V that is 80 cm by 30 cm by 43 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 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 U
= 12 x 12 x 12
= 1728 cm³ 

Volume of V
= 80 x 30 x 43
= 103200 cm³ 

2 ℓ = 2000 cm³ 
3.5 ℓ = 3500 cm³ 

Net increase in volume per minute
= 3500 - 2000
= 1500 cm³/min

Height of T to be filled with water
= 68 - (43 + 12)
= 13 cm

Volume of water in T
= 25 x 25 x 13
= 8125 cm³ 

Volume of water in V, U and part of T
= 103200 + 1728 + 8125
= 113053 cm³ 

Time taken to reach height
= 113053 ÷ 1500
≈ 75 min (Correct to nearest whole number)

Answer(s): 75 min