(1) If you buy 8 clothes in a store, each dress needs 800-20×8=640 yuan, and 8 clothes * * * need 640×8=5 120 yuan; It costs 800× 75 %× 8 = 4,800 yuan < 5 120 yuan to buy in store B, so it should be bought in store B; (2 points)

(2) If the dealer buys X pieces, it will cost y=x(800-20x) yuan (an integer of 0 < X ≤ 18); If you buy in store B, you need to spend Y = 800× 0.75 yuan (an integer with X > 0); (4 points)

(3) (1) If the dealer spends 6720 yuan on clothes in a store,

Then x(800-20x)=6720, x= 12, x=28 does not meet the meaning of the question (2 points).

② If the dealer spent 6720 yuan on clothes in Store B, 600x=6720, and x= 1 1.2 did not meet the requirements, so it was invalid. ( 1)

So, the dealer bought 12 pieces of clothes in a store. (1 min)