Put these in order, smallest first:
0.5
75%
2/3

smallest to greatest: 0.50 , 2/3, 75%

We need to get each number into a common form to easily compare one another. We can use the decimal form.

0.50 = 0.50  *smallest
75% = 0.75  *largest
2/3   = 0.67  *middle

Or we can convert each number to percentage form:

0.50 * 100% = 50%
75%             = 75%
2/3 * 100%   = 67%

We can also convert each number into fractions. Just solve for the common denominator. 

0.5 = 0.5
75% = 0.75
2/3 = 0.6666666...

In order, from smallest to greatest, it would be 0.5, ²/₃, 75%.

Your final answer is 0.5, ²/₃, 75%.