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RD Chapter 16 Surface Areas and Volumes Ex 16.3 Solutions

Question - 11 : - A bucket is in the form of a frustum of a cone of height 30 cm with radii of its lower and upper ends as 10 cm and 20 cm respectively. Find the capacity and surface area of the bucket. Also, find the cost of milk which can completely fill the container, at the rate of Rs.25 per litre.

Answer - 11 : -

Let R and r be theradii of the top and base of the bucket respectively,

Let h be its height.

Then, we have R = 20cm, r = 10 cm, h = 30 cm

Capacity of the bucket= Volume of the frustum of the cone

= 1/3 π(R2 +r2 + R r )h

= 1/3 π(202 +102 + 20 x 10 ) x 30

= 3.14 x 10 (400 + 100+ 200)

= 21980 cm3 =21.98 litres

Now,

Surface area of thebucket = CSA of the bucket + Surface area of the bottom

= π l (R + r) + πr2

We know that,

l = √h2 +(R – r)2

= √[302 +(20 – 10)2] = √(900 + 100)

= √1000 = 31.62 cm

So,

The Surface area ofthe bucket = (3.14) x 31.62 x (20 + 10) + (3.14) x 102

= 2978.60 + 314

= 3292.60 cm2

Next, given that thecost of 1 litre milk = Rs 25

Thus, the cost of21.98 litres of milk = Rs (25 x 21.98) = Rs 549.50

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