Find the expected value of the number of tests taken.
A calculus instructor uses computer aided instruction and allows students to take the midterm exam as many times as essential until a passing grade is obtained. Following is a record of the number of students in a class of 20 who took the test each number of times.
Students

Number of tests

10

1

7

2

2

3

1

4

Find out the expected value of the number of tests taken.