问一道prep数学题，谢谢大家啊

刚模了一套Prep，有道数学没看懂……

Before being simplified, the instructions for computing income tax in Country R were to add 2 percent of one's annual income to the average (arithmetic mean) of 100 units of Country R's currency and 1 percent of one's annual income.  Which of the following represents the simplified formula for computing the income tax, in Country R's currency, for a person in that country whose annual income is I ?

答案是C, 50+I/40，怎么也不明白，拜托看懂的同志帮我解释一下题~感激ing~~

2% * I + (100+ 1% * I)/2 =50+ 5/200 * I= 50+ I/40

不好意思  最后一部写错了