SOLUTION: a contractor mixes concrete from bags of pre-mix for small jobs. How many bags with 6% cement should he mix with 4 bags of 21% cement to produce a mix containing 9% cement?

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Question 729166: a contractor mixes concrete from bags of pre-mix for small jobs. How many bags with 6% cement should he mix with 4 bags of 21% cement to produce a mix containing 9% cement?
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
x = how many bags of 6% cement
y = 4 bags of 21% cement
L = 6% cement
H = 21% cement
T = the target 9% cement

The unknown variable is x.
highlight%28%28Lx%2BHy%29%2F%28x%2By%29=T%29
Solve for x. Substitute the values for the formula to evaluate x.