Look at http://www.purplemath.com

I think that you will like it.It is an algebra help site.

Here is a site that they recommend for tutoring

http://www.mathnerds.com/mathnerds/about/about.aspx

Saxon Advanced Mathematics

I don’t know of an online place, but I can help with the type of problem you described. First you have to assign a volume to your solution, it doesn’t matter what so use something easy like 1 liter. Let’s say it is 50% water and 50% acid and you want to add water until it’s 25% acid. You know you have a half liter of acid so how much total solution do you need for a half liter to be 25%. You need two liters of solution. Since you already have 1 liter of solution. You would have to add 1 liter of water. This is a simple example but the reasoning behind it can carry over to any problem of this type.

saxon advanced math