Answer :

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


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