【求助】prep上的数学~~~？？？

题目是这样的：

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 units of Country R's currency and 1% of one's annual income, which is the simplified formula? (annual income is G)

answer: 50+G/40

请给解题过程，谢谢大家啊~~~~