The first group is 5 - 9, the second group is 10 - 14 and the third group is 15 - 19. We now find the number of data within the interval 15 - 19. In the given frequency distribution, 17, 15, 18, 19, 19 and 16 are within the interval 15 - 19. Therefore, the frequency of the third group is 6.