<span>The mean of the sample is 50 and the standard deviation is 12. If 5 is subtracted from each of the values the standard deviation will remain the same, as the standard deviation is the distance of each value from the mean. Since all the scores are subtracted by 5, the effective distance from the mean is going to be the same but the mean will be reduced by 5.</span>
If you covert this into a mixed fraction it would be 75/4 (4*18 is 72 then add the 3 in the numerator) then divide, 75/4 = 18.75 (sorry if explanation is bad)