Algebra
posted by Jon .
1)A bookshelf holds 4 mysteries, 3 biograpghies, 1 book of poetry, and 2 reference books. If a book is selected at random from the shelf, find the probability that the book selected is a biography or reference book.
A)1/2
B)1/6
C)5/6
D)3/50
I chose A
5 bio/reference/10 total books
1/2
2)Find the standard deviation of the data set to the nearest tenth. (21,13,18,16,13,35,12,8,15)
A)16.8
B)7.8
C)7.3
D)5.7
I chose C
mean = 21+13+18+16+13+35+12+8+15/9
16.8
variance = (2116.8)^2 = 17.64...did that with the rest and added them to get sqrt 53.7
standard deviation = sqrt 53.7
= 7.3
3)Find the exact value of sin165
A)(sqrt6)  (sqrt2)/4
B)(sqrt6) + (sqrt2)/4
C)(sqrt2)  (sqrt6)/4
D)(sqrt of sqrt6  sqrt2)/4
I chose A
(in D, there is a sqrt over both the sqrt6 and sqrt2
If I have it wrong can you explain how to do these types of problems b/c what I do is just take the sqrt of 6, subtract that from the sqrt of 2 and then divide by 4. I always end up with the same answer when I get the value of sin165 when I type it in my calculator

For question one use:
P( A or B) = P(A) + P(B)  P(A and B)
The rule for OR takes into account those values that may get counted more than once when the probability is determined.
Let P = probability
Let A = biographies
Let B = reference book
Can you finish? 
No. why would I not just take the number of bio and ref. and divide by the total number of books?

since P(A and B) from blueridge's formula is zero, you are right, total is 5
so prob. = 5/10 = 1/2