If all possible samples of size n are drawn from an infinite

If all possible samples of size n are drawn from an infinite population with a mean of 54 and a standard deviation of 15, then the standard error of the sample mean equals 3 for samples of size:

Solution

Standard Error= sd/ Sqrt(n)
Where,
sd = Standard Deviation
n = Sample Size

Standard deviation( sd )=15
Sample Size(n)= Unknown
Standard Error = ( 15/ Sqrt ( Unknown ) )
3 = ( 15/ Sqrt ( Unknown ) )

Sqrt ( Unknown ) = 15/3
Sqrt ( Unknown ) = 5
Sample Size = 5^2 = 25