So we have a weapon with probability of working succesfully. We test weapons and the stockpile is replaced if the number of failures is at least one. How large must n be to have ?

a)Use exact binomial

so

b)Use normal approximation

I know we can use ...

And then normalise this distribution to the standard normal form something like:

If that is correct I have to solve that for n,.... how?

Also I doubt that rewriting is smart...