# Simple Probabilities Plz Help

• Apr 23rd 2008, 02:14 PM
ballin_sensation
Simple Probabilities Plz Help
Hey,

Can someone plz answer the following questions and provide explanation.

1. P(E)=0.25 and 20 outcomes make up event E. Determine the number of outcomes?

2. There are 7 girls, 9 boys in a school camera club. The club needs to form a committee of 6 random people. Determine the outcome if:
a) The committee has no boys.
b) The committee has at least 1 girl.

3. You have a regular 6 side die. You roll it 2 times. Determine if:
a) Probability that you roll a sum of 7.
b) Probability that you roll a sum of 7 if the first roll is 5.

Any help would be greatly appreciated.

Thanx
• Apr 23rd 2008, 04:24 PM
lllll
1. $\displaystyle 20 \times 0.25 = 5$ I'm not 100% sure about this one.

2. so there are a total of 16 students and out of them you are interested in choosing six.

$\displaystyle \binom{16}{6} = 8008$ possible picks.

a) $\displaystyle P(X=0) = \frac{\binom{9}{0} \binom{7}{6}}{8008} = \frac{7}{8008}$

b) $\displaystyle P(Y \geq 1) = 1- P(Y=0) = 1- \frac{\binom{9}{6} \binom{7}{0}}{8008} = 1- \frac{84}{8008} = \frac{283}{286}$

3. your sample would consist of $\displaystyle S= (1,1), (1,2), ... , (6,6)$

a) so out of those there are 6 possible ways to get a 7 and $\displaystyle 36= 6\times 6$ outcomes. so you have:

$\displaystyle \frac{6}{36} = \frac{1}{6}$

b)$\displaystyle P(7|5) = \frac{P(7 \cap 5)}{P(5)} = \frac{1/36}{1/6} = \frac{1}{216}$
• Apr 23rd 2008, 04:33 PM
ballin_sensation
Thanx for the help llll and mathstud.
• Apr 23rd 2008, 04:37 PM
ballin_sensation
shoudnt the answer for 3b) equal 1/6 not 1/216???
• Apr 23rd 2008, 04:50 PM
Plato
I really wish that if you do not understand the process that you would reframe from giving totally wrong information.
If “P(E)=0.25 and 20 outcomes make up event E” then clearly E happens in ¼ of the experiences, so there would have been 80 experiences.
• Apr 23rd 2008, 05:40 PM
lllll
Quote:

Originally Posted by ballin_sensation
shoudnt the answer for 3b) equal 1/6 not 1/216???

you are correct it should be 1/6, I made a mistake punching it in my calculator.