An ointment is prepared by incorporating $10\,\mathrm{g}$ of a drug into a $100\,\mathrm{g}$ of white petrolatum. What is the percent w/w of active ingredient?
The answer is $9.1\,\%$. I don't know how it came up with the answer because I always get $10\,\%$. Thank you for answering!
You have $10\,\mathrm{g}$ of a drug and $100\,\mathrm{g}$ of white petrolatum, so $110\,\mathrm{g}$ in total.
So, you have $$\frac{10}{110} \cdot 100\,\% = 9.1\,\%$$ of a drug.