请教一个statistics 的问题

General Hospital’s patient account division has compiled data on the age of accounts receivables. The data collected that the age of the accounts follows a normal distribution, with mean 28 days and a standard deviation 8 days.

Question:

The hospital administrator would like to give a 5% discount to those accounts that pay their balance by the 21^st day. What percentage of the accounts will receive the discount?

我自己也做了一下这个题目,但是结果跟正确的答案不一样, 正确的是14.81%. 我的答案是18.94%

请教统计高手进来教我一下,请告诉我详细的过程.