The algorithm of this problem is that a shirt is 400 yuan, and the second one is 200 yuan at half price, that is to say, two pieces should be 600 yuan, with an average of one 300 yuan, and then the price of 300 yuan divided by one piece is 400 yuan, and the result is equal to 0.75, so this purchase method should be equivalent to 7.5 discount on two pieces, 800 times 0.75, which is exactly 600 yuan, so that we can apply mathematics everywhere in our lives, learn mathematics well and do business, and then we can settle accounts.