Easy way to calculate the amount of pure silver in pre 1965 US Silver Numismatic coins.

One day while making my small weekly silver purchase at my local coin dealer, Larry, (the proprietor) told me of an easy way to figure the pure silver weight in numismatic coins.
Example: Lets say I have 1 quarter and 1 dime equaling 0.35 cents. Divide 0.35 by 1.40 equals 0.25 or a 1/4 ounce of pure silver.
0.35 / 1.40 = 0.25
This works with any combination or amount of face value.
I hope that helps you figure out how much pure silver is in your stack. ;)