A common denominator can also be found for any group of fractions by multiplying all the denominators together. This number is not always the LCD, but it can still be used to add or subtract fractions.

A common denominator can be found by multiplying the

In order to add fractions, the denominators must be the same number. This is referred to as having “common denominators.”

Example: Add 1⁄7 to 3⁄7

If the fractions do not have the same denominator, then one or all the denominators must be changed so that every fraction has a common denominator.

Example: Find the total thickness of a panel made from 3⁄32-inch thick aluminum, that has a 1⁄64-inch thick paint coating. To add these fractions, determine a common denominator. The LCD for this example is 1, so only the first fraction must be changed since the denominator of the second fraction is already in 64ths.

Therefore, 7⁄64 is the total thickness.

To subtract fractions, they must have a common denominator.

Example: Subtract 2⁄17 from 10⁄17

If the fractions do not have the same denominator, then one or all the denominators must be changed so that every fraction has a common denominator.

Example: The tolerance for rigging the aileron droop of an

which the aileron can be rigged? To subtract these fractions, first change both to common denominators. The common denominator in this example is 40. Change both fractions to 1⁄40, as shown, then subtract.