To find a common denominator you have to find a number that both can be divided by in this case, 15. So both can be set to 15. However the numerators must also be changed. Since 15 divided by 5 is 3, the numerator 2 should be multiplied by 3 to get 6. Then since 15 can be divided by 3 to 5, you multiply 5 to the numerator 1 to get 5.
Then do
6/15 - 5/15 to get 1/15
