Question

If the population standard deviation is 10 , how large a sample is necessary to have a standard error that is

Answer 1

The standard error given a population standard deviation of $\sigma$ is $\dfrac\sigma{\sqrt n}$. So if the desired standard error is $s$, then

$s=\dfrac\sigma{\sqrt n}\implies \sqrt n=\dfrac\sigma s\imlies n=\dfrac{\sigma^2}{s^2}$