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a store is selling two mixtures of coffee beans in one-pound bags. the first mixture has 12 ounces of sumatra combined with 4 ounces of celebes kalossi, and costs $15. the second mixture has 4 ounces of sumatra and 12 ounces of celebes kalossi, and costs $21. how much does one ounce of sumatra and one ounce of celebes kalossi cost?

X = price Sumatra, y = price Celebes

12*x + 4*y=15

4*x + 12*y = 21

Now, solve it. For instance (there are many ways):

Multiply the second equation by (-3):

12*x + 4*y=15

-12*x  -36*y = -63

Sum them up (x cancel out. That's why we multiply by -3)

-32y=-48,  y = 48/32=6/4=3/2

y = 3/2 and x = go to any equation. Say the second again:

4*x + 12*y = 21

4x + 12*3/2 =21 > 4x + 18=21, 4x = 3, x = 3/4.

So cost of Sumatra $ 3/2  per oz, or $ 1.50

Cost of Celebes Kalossi (what a name!) $ 3/4 per oz = $ 0.75 per oz

Let's check it with the first equation:

12*0.75+4*1.5 = 15!

Clear?

He was paid 23 dollars for each lawn