SOLUTION: A contractor mixes concrete from bags of pre-mix for small jobs. How many bags with 7% cement should he mix with 10 bags of 15.5% cement to produce a mix containing 12% cement?

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Question 324939: A contractor mixes concrete from bags of pre-mix for small jobs. How many bags with 7% cement should he mix with 10 bags of 15.5% cement to produce a mix containing 12% cement?
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Let the number of 7% bags be Y.
7Y%2B15.5%2810%29=12%28Y%2B10%29
7Y%2B155=12Y%2B120
-5Y=-35
Y=7
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7 bags of 7% cement need to be added.