I've looked at this problem over and over again and solved it in the most logical way possible, and my professor keeps counting it wrong.

The problem is:

An urban city imposes a sales tax of 7.5% on the cost of consumer's purchases at a retail store. Find the total cost including sales tax for purchases of D dollars. Find the value of the goods purchased if the total bill was $225.


My strategy was D dollars * .075 = tax sum
tax sum + D dollars = total


225 * .075 = 16.875
225 - 16.87 = 208.13


Any help would be great, I'm stumped.