Since 1965, U. S. quarters have been made of copper and nickel and weigh 5.76 grams
each. Before 1965, quarters were made of
silver and weighed 6.25 grams each. How
much less does $15.00 in quarters weigh
today than before 1965?

The answer is 29.4 grams.
How to solve:

1. Convert $15 to quarters. You can do this by multiplying 15 by 4. You use 4 because 4 quarters equal to $1. This will equal to 60 quarters.

2. Multiply both 5.76 and 6.25 by 60 to find the weight of $15 in quarters for today and before 1965.
Today: 5.76 g x 60 = 345.6 grams
Before 1965: 6.25 g x 60 = 375 grams

3. Now that you have the weight of quarters for both time periods, subtract! 375 grams - 345.6 grams = 29.4 grams