I would like to ask how to calculate the z-score of a Binomial test? I am using this http://www.elderlab.yorku.ca/~aaron/Stats2022/BinomialTest.htm link as a reference.

I don't get its assumption (second sentence of step 2). I've searched about the formula for this but still I don't get it. I don't know where I can get the mean value. I got X = 26, n = 30. z = (X-u)/n

Please help me. I'm trying to make my own one-tailed binomial test. (Or the equivalent of one-tailed test in the link I've given above is just the removal of negative critical region???)