Not to get too technical but...
Weight of pennies in lbs. x 453.59 (# of grams in a pound) = Total grams of all pennies
Total grams of all pennies x your copper percentage = Total grams of copper cents
Total grams of all pennies - Total grams of copper cents = Total grams of zinc cents
Total grams of copper / 3.11 = Total # of copper cents
Total grams of zinc / 2.5 - Total # of zinc cents
Total # of copper cents x .01 = Face value of copper cents
Total # of zinc cents x .01 = Face value of zinc cents
Face value of copper cents + Face value of zinc cents = Total Face Value / Fair Price
Example: You want to buy 10 lbs of pennies and you believe %18 of them are copper
10 x 453.59 = 4,535.9
4,535.9 x %18 = 816.462
4535.9 - 816.462 = 3,719.438
816.462 / 3.11 = 262.53
3,719.438 / 2.5 = 1487.78
262.53 x .01 = $2.63
1487.78 x .01 = $14.88
$14.88 + $2.63 = $17.51
This will get you close to a fair face value price.
I've calculated everything from 1 lbs of pennies at %1 copper to 20 lbs at %30 copper and everything in between and fasteddy's # of $1.75 per pound is pretty spot on. (Nice work eddy!!! Old pro!!) $1.75/lb. is going to get you pretty close to within a dollar of face value, (sometimes higher, sometimes lower) regardless of what the percentage is. Keep in mind that as the % of copper gets higher (upwards of %30), the amount you're going to lose using the $1.75/lb. formula is going to increase. (Coppers weigh more than zincs therefore the higher the % of copper, the fewer cents there are making up the total weight) If you look at a pile of cents and it looks like there is a lot of copper, you may want to back off that $1.75/lb. a few cents....Or bump it up, it can be looked at from both ways i suppose...Like fasteddy said, take a look at the cents. If it looks like a typical mix, you can't go too wrong with the $1.75/lb. formula.
Disclaimer: Shifty51 is not responsible for overpayment on unsorted pennies or any financial loss associated with the use of this system...
