How to Add Fractions with Unlike Denominators Using a Calculator

Are you trying to remember how to add fractions with unlike denominators? You know there's a trick to adding fractions, but you also know it's not straightforward. I'll show you a way to work with the numbers that works every time. If you do your calculations with a calculator it's even easier.

Difficulty: Moderate

Instructions

Things You'll Need:

• A calculator
• Pencil and paper

1. Convert the first fraction to an equivalent (equal) fraction by multiplying its numerator (top number) and denominator (bottom number) by the other fraction’s denominator. In this example, the problem will be 2/13 + 7/9. Enter 2 x 9 and then 13 x 9. Write your new fraction down (18/117).

2. Convert the second fraction to an equivalent fraction by multiplying its numerator and denominator by the first fraction’s denominator. In the problem above, 2/13 + 7/9, enter 7 x 13 and 9 x 13. Write this new fraction down. (91/117).

3. Add the numerators of the two new fractions. In the example, 18+91=109. This is the numerator for your answer. The denominator is the one that was common to both new fractions (117). So the solution to 2/13 + 7/9 = 109/117. And that’s all you need to do to find a correct answer! If you also want tips on how to reduce fractions that you get for solutions, see tips below. Good luck!

Tips & Warnings

• If the answer you get is too large for practical use, divide the numerator and denominator by a number. If your answers are both whole numbers, you’ve found an equivalent fraction. If you come up with a decimal for one or both of the answers, guess a different number to divide by. For example, if you get 9/36 as a solution, try dividing 9 by 2 and 36 by 2. You’d get 4.5/18. That doesn’t work because you get a decimal in a fraction. If you guess and try dividing 9 and 36 by 4, 5, 6, 7, or 8, you end up with decimals too. But if you guess and try 9 divided by 9 then 36 divided by 9, you get 1/9, which is the equivalent of 9/36.