Source: p 603, A Concise Introduction to Logic (12 Ed, 2014) by Patrick Hurley
A similar misuse of percentages is committed by businesses and corporations that, for whatever reason, want to make it appear that they have earned less profit than they actually have. The technique consists of expressing profit as a percentage of sales volume instead of as a percentage of investment. [...] To appreciate the fallacy in this procedure, consider the case of the jewelry merchant who buys one piece of jewelry each morning for $9 and sells it in the evening for $10. At the end of the year the total sales volume is $3,650, the total investment $9, and the total profit $365. Profits as a percentage of sales amount to only 10 percent, but as a percentage of investment they exceed 4,000 percent.
Why is the bold true?
I cannot diagnose why, but I somehow believe that because the merchant buys the 1 piece each morning for 365 days, his total investment should be $9 x 365. Please correct my incorrect thinking.