According to the meaning of the question, the number of people participating in the long jump, high jump and race is 50, 40 and 30 respectively, totaling 120. In order to minimize the number of people participating in more than one project, the number of people participating in only one project should be as large as possible, and the number of people participating in more than one project at the same time should be as three as possible. So the extreme situation is that there are no people who only participate in two activities, only the number of people who participate in one or three activities. Assuming X and Y respectively, there are X+Y = 100 and X+3Y = 120, and the solution is Y = 10.