One 16-ounce bottle of an energy drink has an average of 400 mg of caffeine with a standard deviation of 20 mg. in a carton containing 25 bottles, what is the standard deviation of the average amount of caffeine

Given:
μ = 400 mg, population mean
σ = 20 mg, population standard deviation.
n = 25, sample size

The standard deviation of the sample is the same as the standard error of the mean, and it is calculated as
σₓ = σ/√n
     = 20/√25
     4 mg

 4 mg