# Thread: 100 coin flips, variance?

1. ## 100 coin flips, variance?

Hello,
I'm studying for a test and came across a problem that I am having a hard time arriving at the correct answer. The question is :Let X represent the number of heads in100 coin flips of a fair coin. The variance of X is? I know the answer to be 25, but I cannot arrive at this answer. I tried messing around with something like P(X = 100) bin(100,0.5) and then using something like variance = np(1-p) but its not working right. Any help is greatly appreciated so I can understand better, thanks!

2. Originally Posted by Mahonroy
Hello,
I'm studying for a test and came across a problem that I am having a hard time arriving at the correct answer. The question is :Let X represent the number of heads in100 coin flips of a fair coin. The variance of X is? I know the answer to be 25, but I cannot arrive at this answer. I tried messing around with something like P(X = 100) bin(100,0.5) and then using something like variance = np(1-p) but its not working right. Any help is greatly appreciated so I can understand better, thanks!
np(1-p)=100(.5)(.5)=25

Problem?