100 surnames were randomly picked up from a local telephone directly and the frequency distribution of the number of letters in the English alphabets in the surnames was obtained as follows: Number of letters:   1-4     4-7     7-10     10-13     13-16     16-19 Number surnames     6       30       40          16           4            4 Determine the median number of letters in the surnames. Find the mean number of letters in the surnames. Also, find the modal size of the surnames.