How do you know the number of part to divide a number line from 0 to 1 into when comparing fractions

To compare fractions you need a common denominator, then you can make a comparison.  You would divide the number line between 0 and 1 (or any 2 whole numbers on the number line) based on that common denominator.  For example... if you were trying to compare 3/4 and 2/8, you would find the common denominator (in this case 8) and convert the fractions...

3/4 = 6/8
2/8 stays as 2/8

Now you would go to the number line between 0 and 1 and divide it into 8 equal pieces.  3/4 would go over the 6th division and 2/8 would go over the second division. 

That would give you a good visual that 2/8 < 3/4

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0              2/8                               3/4               1