The figure shows a tank made up of 3 sections. Cuboid D with a square base of side 25 cm and height 26 cm. Cube E with a side of 12 cm. Cuboid F that is 71 cm by 29 cm by 49 cm. Water from a tap above flows down at a rate of 2.5 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 68 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of E
= 12 x 12 x 12
= 1728 cm³ 

Volume of F
= 71 x 29 x 49
= 100891 cm³ 

1 ℓ = 1000 cm³ 
2.5 ℓ = 2500 cm³ 

Net increase in volume per minute
= 2500 - 1000
= 1500 cm³/min

Height of D to be filled with water
= 68 - (49 + 12)
= 7 cm

Volume of water in D
= 25 x 25 x 7
= 4375 cm³ 

Volume of water in F, E and part of D
= 100891 + 1728 + 4375
= 106994 cm³ 

Time taken to reach height
= 106994 ÷ 1500
≈ 71 min (Correct to nearest whole number)

Answer(s): 71 min