I an missing something when it comes to figuring out Relative humidity. 75 degrees F with a mixing ratio of 18.8 I turned it in but I know I was wrong

Answer 1

If you know you are wrong, would it not behoove you to study a bit more? How about starting here:

http://www.atmo.arizona.edu/students/courselinks/fall12/atmo336/lectures/sec1/humidity.html