A hotel chain wants to learn about the level of experience of its general managers. A random sample of 40 general managers is taken and their experience (in number of years) is recorded. The information is summarised in the following frequency table:                 Experience                (years)     Frequency    (f�)     Midpoint      5,5−8,55,5−8,5 33 77 8,5−11,58,5−11,5 55 1010 11,5−14,511,5−14,5 66 1313 14,5−17,514,5−17,5 1212 1616 17,5−20,517,5−20,5 1010 1919 20,5−23,520,5−23,5 44 2222  The best estimate of the mean number of years of experience, rounded to two decimal places, is   Select one: a.    15,4815,48.   b.    17,9517,95.   c.    7,117,11.   d.    4,244,24.