A grouped frequency table with class intervals of equal sizes using 250-270 (270 not included) as one of the class interval is constructed for the following data:
268, 220, 368, 258, 242, 310, 272, 342, 310, 290, 300, 320, 319, 304, 402, 318, 406, 242, 354, 278, 210, 240, 330, 316, 406, 215, 258, 236
The frequency of the class 310-330 is _________.