Question 85227
x = douglas firs
y = ponderosa pine trees
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x + y = 850
300x + 225y = 217,500
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Solve by eliminating the y's
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1) Multiply the top equation by -225
-225x + -225y = -191,250
300x + 225y = 217,500
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Your y's will cancel when you add everything
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75x = 26,250
x = 350 
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2) plug your x-value back into the top equation
350 + y = 850
y = 500
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You end up with 350 douglass firs and 500 ponderosa pine trees. 
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If you need further explanation, just let me know.