The following table gives the ages in completed years of 141 persons in a town involved in road accidents during a particular year.
Ages (years): 12-15, 16-20, 21-25, 26-30, 31-40, 41-50, 51-70
Frequency: 15, 48, 28, 17, 14, 7, 12

Find estimates of the mean and standard deviation of the ages.

OK so my main problem is finding mid-class values; since the ages are in COMPLETED YEARS the class boundaries would be 12<=x<16, 16<=x<21, etc., giving mid-class values 14, 18.5, 23.5, 28.5, 36, 46, 61.
Is this correct? Cos I got mean=26.6 and the answer is 26.9, and Iím sure my calculations are correct, the problem must be with the mid-class values. I also calculated the standard deviation for these mid-class values and it is way off.