Convert them to the same form. We can pick decimals: 15%=15/100=0.15. 1/5=2/10=0.2.
So now we compare 0.15, 0.015 and 0.2. To make them easier to compare we can multiply them all by 1000 to get rid of the decimal points: 150, 15, 200 (multiplying by 1000 moves the decimal point one place to the right for each zero, so we move it three places to the right inserting zeroes when we run out of places). Now it's easy to compare. The least is 15 (from 0.015) and the greatest is 200 (from 1/5). So the order is: 0.015, 15%, 1/5.