Calculate a sample
 

I have a population of 1000 and I want a 95% confidence level and 5% margin
of error. I need to figure out the sample size that I need to study.
I went on various website that have calculators and I found out that the
size should be 278 but I can't back out the calculation.
The only functions I found were to randomly choose the range of number or to
calculate the confidence level

I am sure it is very easy but I need help !
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Thanks
stephanie