57. (错误原因：没有理解题意，不知道是买了一批半价的衣服还是只有一件；开始按一件算的，没有答案，因为出现了半件衣服)
During a sale, for each shirt that Mark purchased at the regular price, he also purchased a shirt at half the regular price.  How many shirts did Mark purchase during the sale?

(1)  The regular price of each of the shirts that Mark purchased during the sale was $21.50.

(2)  The total of the prices for all the shirts that Mark purchased during the sale was $129.00.

 for each shirt .... he also purchased a shirt at half the

regular price ...

也就是buy one and get one half of the regular price

假设买了x件原价的，那就同时也买了x件半价的