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The "Charming Teacher" student original web article contest conducted in our city had the uploading period from August 1st to 30th. The judging panel grouped the submitted articles from each school into 5-day intervals and created a frequency distribution histogram (as shown in the image). It is known that the heights of the rectangles from left to right are in the ratio 2:3:4:6:4:1, and the frequency of the second group is 180. Then the total number of articles received in this activity is ________. 