answer:I’m not sure where you got “4 parts of 100% plus 95 parts of 1%,” but that adds up to only .04 + .95 = 99%. Let’s say you add x grams of yellow mercuric oxide (ymo). I’m assuming percentages are by mass, so volume is irrelevant. The final total mass of the mixture is 30 + x (because you added x) The initial mass of ymo was (30g)(1%) = 0.3 grams The final mass of ymo is therefore x + 0.3 (because you added x) *** The final concentration is 5%, so final mass of ymo is (30+x)(5%) = (30 + x)(.05) = 1.5 + .05x *** Equating the two starred equations above *** for mass of ymo: x + 0.3 = 1.5 + .05x .95x = 1.2 x = 1.2/.95 = 1.263… grams of ymo added. So the final ointment contains 1.563… g of ymo in a total mass of 31.263… grams, which is exactly 5%.