How to find common denominators

Ok, It's hard to explain so I'll use an example.

1/5 + 2/15

now you try to find a number that goes into both for this example it would be 3

3*5=15 and then that equals the 2/15

so you then multiply 3 by the numerator getting the fraction

3/15 + 2/15

with the answer of

5/15 which can be reduced to

1/3