The figure shows a container made up of 3 sections. Cuboid B with a square base of side 4 cm and height 34 cm. Cube C with a side of 12 cm. Cuboid D that is 68 cm by 28 cm by 50 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 container to a height of 69 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of C
= 12 x 12 x 12
= 1728 cm³ 

Volume of D
= 68 x 28 x 50
= 95200 cm³ 

1 ℓ = 1000 cm³ 
3.5 ℓ = 3500 cm³ 

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

Height of B to be filled with water
= 69 - (50 + 12)
= 7 cm

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

Volume of water in D, C and part of B
= 95200 + 1728 + 112
= 97040 cm³ 

Time taken to reach height
= 97040 ÷ 2500
≈ 39 min (Correct to nearest whole number)

Answer(s): 39 min