find the mean and the standard deviation of X

[nkeyser]

The answer to the question I found is

Mean = 20 Standard Error = 3.464​

I do not know how they came up with this answer, any help would be appreciated. The question is:
Suppose that 40% of men over the age of 30 suffer from lower back pain. For a random sample of 50 men over the age of 30 find the mean and the standard deviation of X (the number of men over the age of 30 that suffer from lower back pain.)

 

[galactus]

They used:

\(\displaystyle mean=np\)

\(\displaystyle \text{standard deviation}=\sqrt{np(1-p)}\)

n=sample size and p = percentage

 

[nkeyser]

Thank you for your help, but I am still getting standard error of 2.683281573.
I have done this over and over and still get this same answer. But it's apparently wrong, unless my instructor posted the answer wrong. Can someone check and see what you get for an answer?

 

[mmm4444bot]

Can someone check and see what you get for an answer?

Using the expressions supplied by galactus, I get 3.464 (rounded).

If you cannot locate your mistake(s), please show your arithmetic, so that we may see what you're actually doing.

 

[nkeyser]

Thank you

Thank you both again, I was putting in the calculator wrong, when I adjusted how I entered it I got the correct answer. I really appreciat all your help