The figure shows a tank made up of 3 sections. Cuboid K with a square base of side 14 cm and height 45 cm. Cube L with a side of 15 cm. Cuboid M that is 60 cm by 17 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 L
= 15 x 15 x 15
= 3375 cm³ 

Volume of M
= 60 x 17 x 43
= 43860 cm³ 

2 ℓ = 2000 cm³ 
3.5 ℓ = 3500 cm³ 

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

Height of K to be filled with water
= 68 - (43 + 15)
= 10 cm

Volume of water in K
= 14 x 14 x 10
= 1960 cm³ 

Volume of water in M, L and part of K
= 43860 + 3375 + 1960
= 49195 cm³ 

Time taken to reach height
= 49195 ÷ 1500
≈ 33 min (Correct to nearest whole number)

Answer(s): 33 min