Question 240719
Setup:
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Equation 1: {{{X + Y = 28}}}
Equation 2: {{{Y = X + 4}}}

Solution:
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Since we know that Y = X + 4, plug (X + 4) into equation 1 for Y
Equation 1: {{{X + Y = 28}}}
{{{X + (X + 4) = 28}}} Simplify
{{{2X + 4 = 28}}} Subtract 4 form both sides
{{{2X = 24}}} Divide both sided by 2
{{{highlight(X = 12)}}}

Now plug 12 in for X into equation 2
Equation 2: {{{Y = X + 4}}}
{{{Y = 12 + 4}}}
{{{highlight(Y = 16)}}}

Checking the answer:
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Now go back to equation 1 and plug in 12 for X and plug 16 in for Y
Equation 1: {{{X + Y = 28}}}
{{{12 + 16 = 28}}}
{{{12 = 12}}}