This question cannot be solved.
From the information given, the ratio of blue, red and magenta blouses are 6,3 and 2 respectively. The total people involved here must be a multiple of 11, unless someone is wearing a multicolored blouse.
Also, it assumes that the 304 people are only wearing red, blue or magenta blouses. What if some of the participants are wearing white, yellow or some other color blouse ?