A contractor mixes concrete from bags of pre-mix for small jobs. How many bags with 3% cement should be mix with 5 bags of 30% cement to produce a mix containing 12% cement?

(3*b+30(5))/(b+5) =12. Cross-multiplying, 12(b+5)=3b+30*5; 12b+60=3b+150; 9b=90; b=10.

Check work: (3*10+30(5))/15 is (30+150)/15 is 180/15 is 12 as desired. 10 bags.

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