Nucor Corporation produces steel and steel products at its eight mills and is a major recycler of scrap metal. The following data relate to Nucor for four years. In 2017, Nucors net income was higher by $175.2 million because of a one-time effect of the Tax Cuts and Jobs Act.

Nucor Corporation
($ in millions)	2015			2016			2017			2018
Total assets	$	14,327.0		$	15,223.5		$	15,841.3		$	17,920.6
Common stockholders equity		7,849.9			8,254.7			9,084.8			10,202.0
Total debt		4,388.5			4,357.1			3,795.1			4,291.1
Sales					16,208.1			20,252.4			25,067.3
Net income					900.4			1,380.6			2,481.1
Interest expense					169.2			173.6			135.5
Assumed marginal income tax rate					0.37			0.37			0.23

Source: Nucor Form 10-Ks for 20152018.

Required:

In the following analyses, eliminate the one-time net income effect of the Tax Cuts and Jobs Act on 2017s results.

I'm having a lot of trouble answering these, and there are a lot of wrong answers around Chegg, so don't copy them because that will only make me only more confused. Please answer the questions below, in terms of percentage, or just tell me how to calculate the percentages easily.

1. Calculate Nucors ROA for 2016, 2017, and 2018.
2. Decompose ROA into operating profit margin and asset turnover components.
3. Has Nucors profitability changed over the three years? <- According to my calculations this one is a yes, so don't worry about answering it.
4. Calculate the rate of return on common stockholders equity for 2016, 2017, and 2018.