Say we want to add:
The denominator of the first fraction is 3. The denominator of the second fraction is 5. Because these 2 numbers are different, we need the lowest common denominator, which, in this case is 15 because 3 x 5 = 15. So we multiply:
and
We can do this because 5/5 = 1 and 3/3 = 1 and we know that multiplying a number by 1 doesn't change the number. Now we add:
