A container open at the top, is in the form of a frustum of a cone of height 24cm with radii of its lower and upper circular ends as 8cm and 20cm respectively. Find the cost of milk which can completely fill the container at the rate of 21 per litre. [Use π=22/7]

Given:
h=24cm
Upper radius = r1 = 20
Lower radius = r2 = 8
Volume of frustum of a cone = (1/3)πh[r1² + r2² + r1r2]
= (1/3) × (22/7) × 24 [20² + 8² + 20 × 8]
= (1/3) × (22/7) × 24 [400 + 64 + 160]
= (1/3) × (22/7) × 24 × 624
= 15689.14 cm³
= 15.69 litre
The cost of milk which can completely fill the container at the rate of Rs. 21 per litre = 21 × 15.69 = Rs. 329.49