The figure shows a tank made up of 3 sections. Cuboid P with a square base of side 4 cm and height 34 cm. Cube Q with a side of 11 cm. Cuboid R that is 76 cm by 23 cm by 45 cm. Water from a tap above flows down at a rate of 2.4 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 69 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of Q
= 11 x 11 x 11
= 1331 cm³ 

Volume of R
= 76 x 23 x 45
= 78660 cm³ 

1 ℓ = 1000 cm³ 
2.4 ℓ = 2400 cm³ 

Net increase in volume per minute
= 2400 - 1000
= 1400 cm³/min

Height of P to be filled with water
= 69 - (45 + 11)
= 13 cm

Volume of water in P
= 4 x 4 x 13
= 208 cm³ 

Volume of water in R, Q and part of P
= 78660 + 1331 + 208
= 80199 cm³ 

Time taken to reach height
= 80199 ÷ 1400
≈ 57 min (Correct to nearest whole number)

Answer(s): 57 min