If you have 20gm of a 15 percent Coal Tar ointment on your shelf The medical Dr has ordered a 5 percent ointment for your patient How much Vaseline would you need to add to get this?

To reduce the concentration of something in a substrate from 15 percent to 5 percent, you need to either decrease that something by three, or you need to triple the volume of the substrate.

Assuming the coal tar ointment is based on vaseline, or that the density of each is comparable, you would add 40gm of vaseline to the 20gm of ointment, making a total mass of 60gm.