How can I calculate common size percents when dealing with a 3 year income statement? I think I understand the two because you are subtracting the difference and then doing the percentage but I don't understand how to do the three column.

The Book & Magazine Co. has inventory of $193,000, equity of $395,100, total assets of $578,800, and sales of $612,300. What is the common-size percentage for the inventory account?