SOLUTION: a 20-lb bag of cement contains 25% cement and 75% sand. How much pure cement must be added to produce a cement mix that is 40%? I can't access my e-text to look up the procedure

Question 786320: a 20-lb bag of cement contains 25% cement and 75% sand. How much pure cement must be added to produce a cement mix that is 40%?
I can't access my e-text to look up the procedure.

Answer by josgarithmetic(39618) (Show Source): You can put this solution on YOUR website!
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:

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.
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