Question 654077
Remember adding fractions like 1/4 + 1/6?  You have to find a common denominator and the  you can add.  So the above would be 3/12 + 2/12 = 5/12.   The person who started solving this problem used that 12 because 6 and 4 both go into 12.  By multiplying by 12 you get rid of the fractions.
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That being said, you can solve this problem by finding common denominator instead:
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m/6 + m/4 = 1
12 is the common denominator, so:
2m/12 + 3m/12 = 1
5m/12 = 1
Think of this as:
5m/12 = 1/1
And cross-multiply:
5m = 12
m = 12/5