Zip Plus is a company that adds bar codes to US Mail. They charge $100 for the first 500 pieces of mail plus 10 cents for each additional piece. If the total bill was 143.20, how many additional pieces of mail were bar coded?
Let x be the number of additional pieces of mail bar coded.
The total cost for the additional pieces is 0.10x dollars.
The cost of the first 500 pieces is 100 dollars.
The total cost is the cost of the first 500 pieces plus the cost of the additional pieces, so it is 100 + 0.10x = 143.20 dollars.
Subtracting 100 dollars from both sides gives 0.10x = 43.20 dollars.
Dividing both sides by 0.10 gives x = 432 additional pieces of mail. Answer: \boxed{432}.