a ring shape metal plate has an internal radius of 7 cm and an external radius of 10.5 cm . if the cost of material is rupees 5 per sq.cm , find the cost of 25 rings .
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r = inner plate radius
R = outer plate radius
area of ring=area of outer plate -area of inner plate
area of ring = pi.R² - pi.r²
=pi (R² -r² )
=pi (R - r)(R + r)
r =7 cm ,R =10.5 cm
area of ring =pi. (10.5-7)(10.5+7)
=22/7 x (3.5)(17.5)
=192.5 cm²
cost of one ring =192.5 x 5 Rs
cost of 25 rings =192.5 x 125 Rs
=Rs 24062.5
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