SOLUTION: A contractor mixes concrete from bags of pre-mix for small jobs. How many bags with 4% cement should he mix with 4 bags of 24% cement to produce a mix containing 9% cement?

Question 907529: A contractor mixes concrete from bags of pre-mix for small jobs. How many bags with 4% cement should he mix with 4 bags of 24% cement to produce a mix containing 9% cement?
Answer by richwmiller(17219) (Show Source): You can put this solution on YOUR website!
We have 4 bags of 24% cement
We want to add some 4% cement to make 9% cement
0.04*x+0.24*4=0.09(4+x)
0.04*x+0.96=0.36+0.09x
0.04x-0.09x=0.36-0.96
-0.05x=-0.6
x=12 bags at 4% (0.48) is added to 4 bags at 24%(0.96) to make 9% of 16 bags(1.44)
check
0.04*12+0.24*4=0.09(4+12)
0.48+0.96=0.09(16)
1.44=1.44
ok

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