First, find a common denominator between the two fractions. 5 is the denominator of 4/5. 5 goes into 15, 3 times. So, multiple 4/5 by 3/3 and get 12/15. This puts the two fractions as fractions of 15. Now, subtract 12/15 from 17/15 and you get 5/15. If you simplify that by dividing by 5 on the numerator and denominator, you get 1/3. Hope this helps :)
