Posted by Shannon on Sunday, December 5, 2010 at 10:02am.
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, Sunday, December 5, 2010 at 10:04am
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, Sunday, December 5, 2010 at 10:10am
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, Sunday, December 5, 2010 at 11:13am
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, Sunday, December 5, 2010 at 1:20pm
Add up all the grades that aren't B's. Then put that over the total amount of grades.

Math  Shannon, Sunday, December 5, 2010 at 3:51pm
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, Sunday, December 5, 2010 at 5:13pm
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.
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