The first group is 5 - 9 ( 9-5 = 4, so each group is in groups of 4)
The second group would be 10 - 14
The third group would be 15 - 19.
Now find the number of data within the interval 15 - 19.
The numbers above between 15 and 19 are: , 17, 15, 18, 19 and 16 (5 total numbers)
The frequency of the third group is 5.