Question

A metal container, open from the top, is in the shape of a frustum of a cone of height 21 cm with
radii of its lower and upper circular ends as 8 cm and 20 cm respectively. Find the cost of milk
which can completely fill the container at the rate of Rs 35 per litre.

Answer 1

According to question

R = 20 Cm
r = 8 cm
H = 21 Cm

Volume of frustum = pi/3 H (R^2 + r^2 + R*r)
                              = pi/3 * 21 (400 + 64 + 160)
                              = 22 * (624)
                              = 13728 Cm^2
1 Cm^3 = 1/1000 Lit 
Thus 13728 Cm^2 = 13.728 Litre

Answer 2

Solution :-

Radius of the upper end of the container = 20 cm

Radius of the lower end of the container = 8 cm

Container is in the shape of frustum.

Height of the frustum = 21 cm

Volume of the container = 1/3πh[R² + r²+ (R*r)] 

⇒ 1/3*22/7*21*[20² + 8² + (20*8)]

⇒ 1/3*22/7*21*(400 + 64 + 160)

⇒ (462*624)/21

⇒ 288288/21

= 13728 cu cm

As we know that 1 litre = 1000 cu cm

So, 13728 cu cm = 13728/1000

= 13.728 litre

Rate of milk = Rs. 35 per litre

Total cost = 13.728 × 35

= Rs. 480.48

Answer.