Question

A ring shape metal plate has an internal radius of 7cm and an external radius of 10.5 cm .if the cost of material is rupees 5 per square centi metre ,find the cost of 25 rings.​

Answer 1

Area of ring=area of outer plate -area of inner plate

let r is the inner plate radius

R is outer plate radius

then ,

area of ring =pi.R^2 - pi.r^2

=pi (R^2 -r^2 )

=pi (R - r)(R + r)

now put r =7 cm and R =10.5 cm

area of ring =pi. (10.5-7)(10.5+7)

=22/7 x (3.5)(17.5)

=192.5 cm^2

now ,

cost of one ring =192.5 x 5 Rs

hence ,

cost of 25 rings =192.5 x 125 Rs

=24062.5 Rs

hope this helps you