Question 1208989
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A 20-pound bag of Economy brand cement mix contains 25% cement and 75% sand. 
How much pure cement must be added to produce a cement mix that is 40% cement?


I think the equation should look like this:


%•amount + %•amount = %(mixture)

0.25(20) + 0.75(x) = 0.40(x + 20)

Is this correct? 
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        This is wrong.


        I will not explain, why it is wrong.


        I will write a correct solution, instead.



<pre>
Assume you add x pounds of pure cement.


Now the total mass is 20+x pounds; the portion of the pure cement is  0.25*20+x  pounds.


You want

    0.25*20 + x = 0.4*(20+x)  pounds.


It is your setup equation to solve and to find x.


Now I solve it for you step by step.  Watch my steps.


    5 + x = 8 + 0.4x,

    x - 0.4x = 8 - 5,

    0.6x = 3,

       x = {{{3/0.6}}} = {{{30/6}}} = 5 pounds  of the pure cement to add  (rounded).    <U>ANSWER</U>
</pre>

Solved.