问一道数学

scholar

on a purchase of $120 , a store offered a payment plan consisting of a $20 down payment and 12 monthly payment of 10 each .what percent of the purchase price , to the nearest tenth of a percent (看不懂）, did the customer pay in interest by using this plan?
key:  16.7%

mmyiru

这个题是问，如果使用了这个付款方式，那么利息占原价的百分之多少，四舍五入到十分位（小数点后一位）

(20+10*12)=140

所以利息是 140-120=20

20/120=16.7%