Question 932133
Let W = number of White shirts
Let Y = number of Yellow shirts

Since we have two variables, we need two equations.
Equation 1: {{{W + Y = 17}}}
Equation 2: {{{9.95W + 13.5Y = 211.75}}}
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Start by solving equation 1 for one of the variables
Equation 1: {{{W + Y = 17}}}
{{{W = 17 - Y}}}
Plug (17-Y) into equation 2 for W
Equation 2: {{{9.95W + 13.5Y = 211.75}}}
{{{9.95*(17-Y) + 13.5Y = 211.75}}}
Simplify
{{{169.15 - 9.95Y + 13.5Y = 211.75}}}
Combine like terms
{{{169.15 + 3.55Y = 211.75}}}
Subtract 169.15 from both sides
{{{3.55Y = 42.6}}}
Divide both sides by 3.55
{{{highlight(Y = 12)}}}
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Using Y=12, solve for W
{{{W = 17 - Y}}}
{{{W = 17 - 12}}}
{{{highlight_green(W = 5)}}}