Question 786320
How much mixture is needed?  Do you want the solution starting from 20 pounds of cement in the bag?  
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You seem to have a MIXTURE of "cement" at 25% PURE CEMENT and you want to add more PURE CEMENT so that the new mixture is 40 % PURE CEMENT.  HERE is how this would work:


{{{highlight((20*25+w*100)/(20+w)=40)}}}
If you are able to study this formula and understand it, then you are learning.  You would next just need to solve for w.  
The variable, w, here was assigned as the unknown amount of pure cement to add to the 20 pound cement to get your target 40% pure cement.