σ/√n<μ< x +z α/2 σ/√n (Black, 2017). Because of the expanded formula, the mean paytime is between 17.09 days and 19.11 days, expressed mathematically as17.09<μ<19.11. Based on these calculations, the effectiveness of the new billing systemcan now be determined. According to the case, the old billing system showed anaverage billing payment time of 39 days. Also, according to the case, the firm made aclaim that prior to implementing the new system, the average payment time wasestimated to be reduced to <19.5 days. The calculations show, that since the newpopulation mean was less than the upper limit of 19.11 days, the electronic billingsystem was successful in reducing the mean payment time by at least 51%.95% Confidence That µ ≤ 19.5 Days?Unlike the previous calculation that dealt with the average period a company tookto pay its creditors; this calculation is best suited for the probability of the mean paymenttime. If µ ≤ 19.5 days, standard deviation is 4.2 days, Sample (n) is 65; The mean iscalculated at 18.10769231. The computed standard deviation = 3.961230384, this is