In a group of 20 students, each student is registered for at least one of three classes – History, Math and English. 10 students are registered for History, 10 students are registered for Math, and 15 students are registered for English. If only three students are registered for all three classes, how many students are registered for exactly two classes?
Choose the correct choice from the following:
A) 13
B) 10
C) 9
D) 8
E) 7
Each student is registered forat least one of three classesmeans thatthere are no students who are registered for none of the classes.   Total = {people in group A} + {people in group B} + {people in group 9} - {people in exactly 2 groups} - 2*{people in exactly 3 groups} + {people in none of the groups}:   20 = 10+10+15 - {people in exactly 2 groups} - 2*3 + 0--&gt;{people in exactly 2 groups}=9   Answer: 9
<result>C</result>
In a group of 20 students, each student is registered for at least one of three classes – History, Math and English. 10 students are registered for History, 10 students are registered for Math, and 15 students are registered for English. If only three students are registered for all three classes, how many students are registered for exactly two classes?