Question 837343
x, y, and z are the numbers.

{{{x+y+z=42}}}

{{{y=2x}}}, and {{{z=y-3}}}
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Any route you see or find, try.
One I see is {{{z=42-x-y}}} from first equation.
{{{z=y-3=42-x-y}}}, to equate expressions for z.
{{{2y+x=42+3}}}
{{{x+2y=45}}}----- which only uses x and y.
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One of the equations from the description also uses only x and y, so we have this simple system of two variables:
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y=2x
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x+2y=45
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A simple substitution will be best next step.
{{{x+2*2x=45}}}
{{{5x=45}}}
{{{highlight(x=9)}}}-------You have one variable's value solved.
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Also created y=2x from the description, so 
{{{y=2*9=highlight(18=y)}}} ----- another variable found.
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Given from description
{{{z=y-3}}}
{{{z=18-3}}}
{{{highlight(z=15)}}}------the last variable found.