you have to get both parts (the number and fraction) in the form of a fraction with the same denominator. Since anything divided by itself is equal to one (3/3, 5/5, 19/19, etc) we can write the number 1 as the denominator of the fraction divided by itself. Here's what I mean:
1 - 1/5 = 5/5 - 1/5 = (5-1)/5 = 4/5
And then for the next one:
1 - 3/5 = 5/5 - 3/5 = =(5-3)/5 = 2/5