Question 1208989
<pre>
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? 

No, it's NOT.

Initial amount of cement (C): .25(20)
Amount of cement (C) to be added: 100%, or 1(C) 
Resulting CEMENT (C) in final mixture: .4(20 + C)

Cement + Cement = Cement
.25(20) + 1(C) = .4(20 + C) ===> .25(20) + C = .4(20 + C)

Now, solve for C, the amount of cement to be added.</pre>