If you know me, you know I’m not looking for homework answers.

I know the answer to this but I got it by a convoluted method of working out which multiple choice answer was correct.

The question:
How many grams of yellow mercuric oxide must be added to 30g of 1% yellow mercuric oxide ointment to prepare a 5% ointment?

What I did figure: yellow mercuric oxide = 100%
To get 5%, I need 4 parts of 100% plus 95 parts of 1%
therefore; 30g(1%) = 95 parts/99 parts

And there I freeze on how to get to grams needed of 100%.

Is it impossible without knowing the final volume? Or am I just stuck?