Question 499424
y = 2x (given)
3x + y = 20  (given)
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substitute y=2x in the second equation
so, you simply replace the 'y' with '2x'
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3x + 2x = 20
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5x = 20
x = 4
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Now that you have x=4, you can find 'y'.
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y = 2x 
y = 2*4
y = 8
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Now check your work using the other equation.
3x + y = 20
3x = 3*4 =12
y = 8
3x + y = 20
Correct.
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You also can use graphs to see where the two lines intersect (which is (4,8)).
{{{graph(500,500,-10,10,-10,10,2*x,-3*x+20)}}}
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Re-read the question to make sure you answer what was asked.

Answer:
x = 4
y = 8
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Done.