We have the following:
Blue: ??
Brown: 0.33
Green: 0.2
Hazel: 0.17
Gray: 0.03
Since this numbers represent relative frequencies, they all must add up to 1. Let x be the value of blue. Then we have the following equation
Then, if we subtract 0.73 on both sides, we get
So the relative frequency of Blue is 0.27.