Probability and binomial distribution question

[NINHARDCOREFAN]

There was a question on the test with the following information (binomial distribution)

n=10
p=.2

Find the probability that X is :
a. At least 3
b. At most 3

For part a I did P(X>=3)=1-P(X<=2)

For part b I did P(X<=3) :
\sum_{x=0}^3 perm(n, x)*p^x*(1-p)^(n-x)

The last part is (1-p)^(n-x)
For part b, I got an answer that is greater than 1, what did I do wrong?

 

[Tide]

Why are you using "perm?"

 

[ksinclair13]

Disregard. How do you delete your posts? I pressed the delete button once I go to edit, but that doesn't do it...

Anyways, I got an answer of .7717.

 

[NINHARDCOREFAN]

Perm meaning permutation. That's the formula for binomial distribution. Ksinclair, should I disregard your answer?

 

[ksinclair13]

Probably :P

I know that moderators can delete posts, but I thought that you could also delete them yourself, or else my post wouldn't even be up there right now.

If perm (n, x) means n!/(x!(n-x)!), then my answer should be correct ;).

 

[NINHARDCOREFAN]

Wow, this sucks. I wrote it out right on the paper but when I kept calculating I kept switching the x and n-x place so i had a greater answer. I can't believe this, I calculated it like 5 times on the test and still didn't realize the mistake!