# roll a die

• April 9th 2008, 07:36 AM
Pookie
roll a die
Help! If I roll a die 8 times...I'd like to know the probability of getting an odd number.

a) exactly 3 times.
b) at least 7 times.
c) at most 6 times.

using the formula: nCr (p)^r (1-p)^n-r (I couln't figure out how to superscript the appropriate letters. I'm hoping whoever can help me out will know what I'm talking about)
• April 9th 2008, 08:19 AM
roy_zhang
Quote:

Originally Posted by Pookie
Help! If I roll a die 8 times...I'd like to know the probability of getting an odd number.

a) exactly 3 times.
b) at least 7 times.
c) at most 6 times.

using the formula: nCr (p)^r (1-p)^n-r (I couln't figure out how to superscript the appropriate letters. I'm hoping whoever can help me out will know what I'm talking about)

You are trying to apply the Binomial probability formula here, realize that your $n=8$ and the probability to get an odd number is $p(\mathtt{odd})=\frac{1}{2}$. Now you can apply the formula here.

(a) $r=3$, so $P=\binom{8}{3}\left(\frac{1}{2}\right)^3\left(\fra c{1}{2}\right)^5$

(b) $r=7\;\mathtt{or}\;8$, so $P=\binom{8}{7}\left(\frac{1}{2}\right)^7\left(\fra c{1}{2}\right)+\binom{8}{8}\left(\frac{1}{2}\right )^8\left(\frac{1}{2}\right)^0$

(c) Can you do it?

Roy
• April 9th 2008, 08:33 AM
Pookie
YES! thank you so much

for C wouldn't I take 1 and subtract the answer of b ?
• April 9th 2008, 08:35 AM
roy_zhang
Quote:

Originally Posted by Pookie
YES! thank you so much

for C wouldn't I take 1 and subtract the answer of b ?

You got it!