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Find the number of coins of 1.5 cm diameter and 0.2 cm thickness to be melted to form a right circular cylinder of height 10 cm and diameter 4.5 cm.



Each one of those coins is also a cylinder (just a very very short one), and its volume is 
V = πr²h = π(.75)²(.2) = 9π/80 cm³ 
The right circular cylinder has volume V = π(2.25)²(10) = 405π/8 cm³. 

Divide the larger cylinder's volume by the volume of each coin = (405π/8) / (9π/80) = 450 coins 
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As the coins are in the form of thin cylinder
Volume of each coin = π×(0.75)²×0.2 =0.1125π cm³
Volume of melted cylinder = π× (2.25)²× 10 = 50.625π cm³
number of coin required = (volume of melted cylinder)/(volume of each coin)
                                 = 50.625π/0.1125π = 450 coins
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