Posted by Shannon on .
If a grade was drawn at random from the data shown below, what is the probability that the grade is not a B?
Grade Frequency
A 2
B 8
C 11
D 2
F 1
I think it is 1 out of 5 or 4 but I am not sure? Please help :) Thank you much!

Math 
MathMate,
There are 5 possible outcomes, one of which is a B.
So what is P(B), and what is P(not B)?
Hint : P(B) and P(not B) have to add up to 1. 
Math 
Shannon,
Oh ok I think I get it, so P(B)=0.5, 1/2,
and P(not B)=0.5,1/2???
Is that right? 
Math 
MathMate,
Sorry, not really.
Figure out the probability of P(B) which is to get B out of 5 possible choices.
The rest would be easy to figure out. 
Math 
Jen,
Add up all the grades that aren't B's. Then put that over the total amount of grades.

Math 
Shannon,
Ok so it would be P(B)=0.8, the probability that the grade is not a B.?
I hope this is right now, if it isn't I give up....lol ! 
Math 
MathMate,
Don't give up, you're right.... almost!
What Jen showed you to do was
P(not B), which equals 0.8
("...grades that aren't B's...")
there is only one grade B, so it is 1/5, there are 4 grades that are not B, so
P(not B)=4/5=0.8
Sorry if it was too confusing, hope it's more clear now.