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Question

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If old pennies have 95% copper and 5% zinc and new pennies have 2.5% copper and 97.5% zinc. How many of each kind of penny do you need to get 60% copper and 40% zinc?

Answer 1

If you assume that a penny has a volume of .36 cm³

and Copper has a density of 8.94 g/cm³

and zinc has a density of 7.135 g/cm³

Then you let x = number of old pennies and y = number of new pennies

Set their ratios so that the copper ratio is .6

and the ratio of the zinc is .4

If it is to be believed that if x & y are whole numbers, then when you do the equations, you find that for both equations, the ratio of x/y is very close to 1.283 = 1283/1000

So, 1283 old pennies mixed with 1000 new pennies gives the correct ratios for both copper & zinc (by weight)!