This is the concept of median of statistical data, given that Mary was the 13th highest and the 13th lowest in the spelling contest it implies that the number of people was distributed as follows;
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25
This implies that since the median of 25 is 13, then the total number of people in the contest was 25