Question 503698
X = count of old pennies
3.09*X = weight of old pennies
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Y = count of new pennies
2.51*Y = weight of new pennies
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X+Y = 885 pennies
Y = 885-X
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3.09*X + 2.51*Y = 2500.4
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multiply by 100 to remove decimals
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309X + 251Y = 250040
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substitute for Y = 885-X
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309X + 251(885-X) = 250040
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309X + 222135 -251X = 250040
309X -251X = 250040 - 222135
58X = 27905
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X = 481.12
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Recall, we do not have counts but instead have average weights.
So having a result that is not an integer is to be expected.
We can assume X = 481.
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That means
Y = 885 - 481
Y = 404
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Always check your work.
3.09 * 481 = 1486.29
2.51 * 404 =  1014.04
1486.29+1014.04 = 2500.33
Compared to 2500.4, it's close.
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Answer:  There were 481 older pennies and 404 newer pennies.