We are interested in the number of apps students use on their smartphones per month. Assume that the number of apps used per month by smartphone owners is normally distributed but that the population standard deviation is unknown. The mean of the sample means is calculated to be 19 apps per month. What is the probability that sample mean is between 18.5 and 19.5 if the sample standard deviation (of a sample of size 25) is 4?

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 Ons stel belang in die aantal toepassings wat studente per maand op hul slimfone gebruik. Aanvaar dat die aantal toepassings wat per maand deur slimfooneienaars gebruik word, normal verdeel is, maar dat die populasie standaardafwyking onbekend is. Die gemiddelde van die steekproef gemiddeldes word bereken as 19 toepassings per maand. Wat is die waarskynlikheid dat die steekproef gemiddeld tussen 18.5 en 19.5 is as die steekproef standaardafwyking (van 'n steekproef van grootte 25) 4 is?