大家帮我看看这个模考中的题怎么理解？

Before being simplified,  the instructions for computing income tax in Country R were to add 2% of one's annual income to the average (arithmetic mean) of 100 unites of Country R's currency and 1% of one's annual income. Which of the following represents the simplified formula for computing the tax, in Country R's currency, for a person in that country whose annual income is I?
Answer:50+I/40

实在是理解不了，请NN帮忙