问题
填空题
Mary invested $8,400 for 6 months in a certificate of deposit paying 
percent simple annual interest. How much more interest would Mary have received if the interest rate on this certificate had been 9 
percent simple annual interest ？（）
答案
参考答案：21
解析：
根据年单利息的计算公式可得：
[*]
=4200×0.5%=21美元
