An open metal buc

Given: Height of the frustum = h = 21 cm

Radius of lower end of the frustum = r = 10 cm

Radius of upper end of the frustum = R = 20 cm

Now, volume of the frustum = (1/3) × π [R² + r² + Rr] × h

= (1/3) × (22/7) × [400 + 100 + 200] × 21

= 700 × 22

15400 cm³

Now, volume of frustum in liters = 15400/1000 = 15.4 lts.

(∵ 1000cm³ = 1 lt)

Now, cost of milk at ₹ 30 per liter = 15.4 × ₹ 30

= ₹ 462