Answer:
3 people were wearing neither color.
Explanation:
Red: 18/36
Blue: 15/36
Both: 7/36
Neither: ?
Both does not matter here because it does not help us find the answer to neither.
Add the red and blue together:
18/36 + 15/36
33/36
33 people are wearing red and/or blue. That leaves 3 people left over (36 - 33 = 3) who are not wearing either color.