level 1 notes book4 page 72 第五题中，为什么average collection period和average days of receivables居然不是一回事？后面解答里的“has less credit sales outstanding since days of receivables are down, so credit sales are a smaller part of Blodnick's business",这句话如何正确理解？能不能给个严格的数学证明？万分感谢！