Answer:
here is a 63.25% for winning the debate given the student distribution and the chance of success of each type of student.
Explanation:
As the student will be picked at random we can determiante the expected value by doing a weighted average:
[tex]\left[\begin{array}{cccc}$Student&$Return&$Probability&$Weight\\$Sophomore&0.2&0.2&0.04\\$Junior&0.6&0.35&0.21\\$Senior&0.85&0.45&0.3825\\Total&&1&0.6325\\\end{array}\right][/tex]