We are given a table of frequencies of each grade. To calculate the probabilities, we simply divide the reported number by the total number of students (35). So we have the table
Grade Pr(Grade)
A 5/35
B 10/35
C 15/35
D 3/35
F 2/35
As we want to calculate the probability that the student earns a grade D or a F, we simply add the probabilities of each grade. That is
[tex]Pr(D\text{ or F\rparen=Pr\lparen D\rparen+Pr\lparen F\rparen = }\frac{3}{35}+\frac{2}{35}=\frac{5}{35}=\frac{1}{7}[/tex]
we also have that
[tex]\frac{1}{7}=0.142857143[/tex]
which rounded to the nearest hundredth is 0.14. So the probability that a student earns a grade of D or F is 0.14
