# When To Use An Alligation?

by Newbie Tech
(St. Paul)

I'm confused as to when to use an allegation or when to use a ratio/proportion pattern when reading a test question. I am a newly hired tech in training for the PTCB and want to know if there are key words/phrases that will help me to use the right calculation pattern.

### Comments for When To Use An Alligation?

 When to use an Alligation by: Keith An Alligation is used primarily when you are going to compound a substance and need to dilute it, either with a lesser strength of the same substance or a straight diluent (i.e.- water) Basically, like most other math, you have a couple of known variables, and are looking for the unknown variable(s). In the case of an Alligation, you must know the strength you want, and then the strengths you have in stock to mix with. One of the ingredients you have in stock to make the final product must be stronger than the desired strength. Remember, you will most always have 0% to work with (diluent), so don't get confused if you're only given one strength to work with.

 Question about 0% by: Anonymous When you say "almost always you will have 0% to work with", does this mean that if they only give one strength, then the "unknown one" will always be 0%?