Clearly, 3/5 is greater than 3/6. You may figure this out mathematically or you may figure it out simply by logic.
By logic, look at what a these fractions mean. In three fifths, you are taking three and dividing it five times. In 3/6, you are taking the same three and dividing it six times. If you cut a pie in five pieces, the slices will be bigger than if you cut it in six pieces. Therefore 3/5 is greater.
You can figure this mathematically as well. The least common multiple of 6 and 5 is 30 so we will make both into fractions with 30 on the bottom.
We multiply the first fraction by 6 and get 18/30 and we multiply the second by 5 and get 15/30. 18 is more than 15, so the first number is greater.