100 surnames are selected at random from a local telephone directory, and the frequency distribution of the number of letters from the English alphabet in the surnames is obtained as follows.

Number of letters	1-4	4-7	7-10	10-13	13-16	16-19
Number of Surnames	6	30	40	16	4	4

Reference: [Number of letters, Number of surnames]

Find the median number of letters in surnames.