Answer:
 The probability of an eighth grader winning is 1/3
Explanation:
Total no. of students = 42 
Number of students in 7th grade = 21
Number of students in 8th grade = 14
Number of students in 9th grade = 7
Probability of event = 

So,the probability of an eighth grader winning= 

Hence The probability of an eighth grader winning is 1/3