The key here is to find the common denominator. In this case 15 is good. 1/5 is 3/15. 1/3 is 5/15. So together, they're 8/15. You could write the answer 5 8/15. If you need to write it in a single fraction, you can convert the 5 to 75/15 and add that in as well, getting you to 83/15.
Hope that helps.
