# A little stuck

• September 16th 2012, 08:23 PM
Past45
A little stuck
I have a problem doing the Binomial Probability. Yes, I know the formula, P(x)=n!/(n-x)!x! yada yada yada.

Now I am I watching the video, it is telling me that 12!/5!(7!), I should be getting 792, for my first step. And I inputed this in my TI84 I get 2.01180672e10..... Where did I go wrong? I inputed the factorial symbol but yet I got it wrong.
• September 16th 2012, 08:28 PM
MarkFL
Re: A little stuck
You need to input 12!/(5!*7!)...your calculator correctly interpreted your original input as (12!/5!)*7!

You need to enclose the denominator within parentheses, and you will get 792 as the result.
• September 16th 2012, 08:30 PM
SworD
Re: A little stuck

12!/5!(7!)

Is actually equal to

$\frac{12!}{5!} \cdot 7! =$ that big number you mentioned

When what you want is

$\frac{12!}{5! \cdot 7!} = 792$

Always remember that parentheses may be needed to make clear what you're trying to do.
• September 16th 2012, 08:31 PM
Past45
Re: A little stuck
I C, thank you, the video did not have those parentheses.
• September 16th 2012, 09:06 PM
Past45
Re: A little stuck
Okay, I am actually stuck on this problem.

Using the Binomial Formula.

N=3, X=2 P=0.60, there is no Q for this one.

After plugging my info, this is what I got.

3!/(2! x 2!) Times p^x, which is 0.60^2..

For 3!/(2!x2!) I got 1.5... For 0.60^2 I got.36... Which I multiplied 1.5 with .36 I got .54... The answer is suppose to 0.432.. Just wondering where I went wrong. TY

Yeah, I still see my error, but the calculation still came out wrong.
• September 16th 2012, 09:31 PM
MarkFL
Re: A little stuck
For Q you would use $Q=1-P$ and your probability would be:

$P(X)={3 \choose 2}(0.6)^2(0.4)=0.432$
• September 16th 2012, 10:20 PM
Past45
Re: A little stuck
Thanks, the book didn't even mention that that it becomes Q=1-P.

Thanks again.
• September 16th 2012, 10:31 PM
MarkFL
Re: A little stuck
This comes from the fact that it is certain that event X will either occur or it will not occur. If we let P be the probability that event X occurs and Q be probability that event X does not occur, then we may state:

Q + P = 1 or Q = 1 - P.

This is sometimes referred to as the complementation rule:

P(E) = 1 - P(not E)
• September 17th 2012, 10:49 AM
Past45
Re: A little stuck
Thanks again. For some reason, I am doing all the right steps on a different problem and my answer is wrong for this problem.

N=8, X=5,P=.90

After doing my plugging in and etc, I have 8!/(3!X5!) *(.90)^2(.1)... I did the right steps didn't I? The answer was suppose to be 0.033, which is clearly off from what I had(4.536).
• September 17th 2012, 11:17 AM
MarkFL
Re: A little stuck
The binomial probability formula is:

$P(x)={n \choose x}p^x(1-p)^{n-x}$

So, you want:

$P(5)={8 \choose 5}(0.9)^5(0.1)^3\approx0.033$