Which fraction is bigger 1/2 1/8 1/3?

The numerator is the same in all three fractions but the denominater isn't. We are going to change that in order to be able to compare the fraction regarding the 'less-than relation' or the 'greater than relation' respectively. We have to convert the fractions to a common denominator. We will do this by multiplying the original fractions with instances of the equivalence class of 1:

1/2 times 12/12 equals 12/24
1/8 times 3 / 3 equals 3/24
1/3 times 8 / 8 equals 8/24

Now you still have to sort the fractions by their numerators:
12/24 > 8/24 > 3/24, hence
1/2 > 1/3 > 1/8

and you're done.