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Formulas:If mu,sigma are the mean and standard deviation of the population. If we take n samples out of the population then sample mean is mu_( bar(X))=mu and sample standard deviation is sigma_( bar(X))=(sigma)/(sqrtn)Facts :If X,Z be the normal and standard normal distibuted random variables respectively.To find the probabilites for X we convert X to Z by using Z=(X-mu)/(sigma)Where mu=mean(X)sigma= standard deviation (X)Let X= The time (in minutes) spent texting in one-month period.Given thatX follows an unknown distribution with mean (mu)=173and standard deviation (sigma)=66.bar(X) is sample mean(Sample average) ... See the full answer