问题如下：
Steve is considering investing PZ Inc. and he asks three of his analysts to forecast the 2018 income statement for PZ. Based on the following information and Peter’s key assumptions, Peter’s forecast for selling, general, and administrative expenses in 2018 is closest to:
选项：
A.$46.4 million
B.$55.5 million
C.$33.1 million
解释：
A is correct.
考点：Income Statement Modeling: Operating Costs
解析：A是正确的。Peter假设2018年SG&A/net sales的比率等于2015- 2017年SG&A/net sales的平均值，并且销售额将以2015- 2017年期间销售额的年均增长率增长。因此，预计2018年的SG&A为4640万美元。
请问一下，答案里面第一列数据，average SG&A/ net sales， 2015-2017， 35% ，34.1%， 36.67% 数据是如何计算的？
