I need HELP with MATH.

by Mona
(Portland, OR)

For the life of me, I can't get the equation (IS)(IV)=(FS)(FV).
it's difficult for me for some reason to tell apart the difference between an alligation math question, or a solution question. I know pretty much all I need to know on my test, and the math part is my brick wall that I just cannot get past, can anyone give me tips on better figuring out these equations and telling the difference! Oh, and here's the math question I need help with:

If 200mL of sterile water for injection is added to an 85mL of an 81% stock solution, what is the percentage strength of the resulting solution?

A. 16.42%
B. 24.16%
C. 34.43%
D. 57.16%

Reply
by Keith
(Phoenix, AZ)

Lets, get something clear for you.

Alligation is only used when you :

1. Know what soluton you are trying to mix.
2. Have a higher strength and a lower strength to work with.

As such, the question above does not fit into those requirements.

Instead of (IS)(IV)=(FS)(FV), I use C1V1=C2V2, it's the same thing, and so whichever you can remember is best for you.

I recorded a video on YouTube about this kind of basic algebra. It's about 12 minutes long and I explain how to work through
(IS)(IV)=(FS)(FV) type equations. In the video, you'll see me use C1V1=C2V2, but just keep in mind that it's all the same.

The video is at this link, which opens a new window

And, don't worry...
The math is going to be easy once you learn how to do it.

Watch the video, and then try to work the above problem.
You should get this equation line:

(85)(.81)+(200)(.00) =(285)(X)