(1) Method 1: Let the number of mountaineering group be X, and the proportions of young people, middle-aged people and old people in swimming group be A, B and C, respectively, which are =47.5% and = 10%, and the solution is B = 50% and C = 10%. Therefore, A =

Method 2: If the total number of people participating in the activity is X, and the proportions of young people, middle-aged people and old people in the swimming group are A, B and C respectively, then the number of young people participating in the mountaineering group plus the number of young people participating in the swimming group is equal to the number of young people participating in the activity, that is, X 50%+X A = X 42.5%, and the solution is a=0.4=40%. Similarly, B.

(2) In the swimming group, the number of teenagers selected is 200×40% = 60; The number of middle-aged people selected is 200×× 50% = 75; The number of elderly people is 200× × 10%= 15.