The figure shows a tank made up of 3 sections. Cuboid P with a square base of side 25 cm and height 20 cm. Cube Q with a side of 13 cm. Cuboid R that is 72 cm by 11 cm by 48 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 tank to a height of 67 cm from the base? Give the answer in minutes correct to nearest whole number.
Volume of Q
= 13 x 13 x 13
= 2197 cm³ 

Volume of R
= 72 x 11 x 48
= 38016 cm³ 

1 ℓ = 1000 cm³ 
3.5 ℓ = 3500 cm³ 

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

Height of P to be filled with water
= 67 - (48 + 13)
= 6 cm

Volume of water in P
= 25 x 25 x 6
= 3750 cm³ 

Volume of water in R, Q and part of P
= 38016 + 2197 + 3750
= 43963 cm³ 

Time taken to reach height
= 43963 ÷ 2500
≈ 18 min (Correct to nearest whole number)

Answer(s): 18 min