Mr. Chang buys supplies for the school store. He bought 300 pencils on sale for $10. During the first three months of school, he sold 200 pencils and made $50. The other pencils were thrown away because they were broken. How much profit did Mr. Chang make on the pencils?
He bought the pencils for $10, so his expense was $10. He sold the pencils for $50, and $50 - $10 is $40, and that's the total amount of money he made.
