<PRE><B>Question 512728</B>
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? 
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Equation:
cement + cement = cement
0.25*20 + 1.00x = 0.40(20+x)
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Multiply thru by 100 to get:
25*20 + 100x = 40*20 + 40x
60x = 15*20
x = 5 lb (amt. of pure cement needed)
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Cheers,
Stan H.</PRE>