# How To: Reduce a fraction by finding common factors in math

The aim of the video is to show how to reduce a fraction by factoring out prime numbers that are common to both the numerator and the denominator. The example shown in the video is 27/21. The number 27 can be achieved by multiplying 3 by 9 and the number 21 is the result of 3 multiplied by 7. The common factor, therefore in both the numerator and the denominator is 3. 3 on the top divided by 3 on the bottom is 1 and so this can be removed without affecting the value of the fraction, leaving 9/7. Therefore, the original fraction of 27/21 can be reduced to 9/7.

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