(1)A unit price: 60 (buy m pieces) b unit price: 90 (buy n pieces) so the result is: 60m+90n (yuan).

(2) Let A buy X clothes and B buy Y clothes, and get the following relationship:

① X+y=60

② 60X+90y=4200

We can get 3x = 60-y from ①, and if we substitute ③ into ②, we can get the following results: Y = 20 (the number of clothes in B), then X = 40 (the number of clothes in A).

(3) Let A buy X clothes and B buy Y clothes, and the total profit is Z, then the following relationship is obtained:

① y=70

② 2X+2=y

③ 20X+30y=Z

Solution: X=34 y=70 Z=2780.