The incomplete table above summarizes the number of right- and left-handed students by grade at Warren Junior High School. None of the students are both right- and left-handed. The ratio of right-handed 7th graders to left-handed 7th graders is 21 to 2. There are 4 more left-handed 8th graders than left-handed 7th graders. If an 8th grade student is chosen at random, what is the probability that the student is right-handed?