Question 162161
Let x=number of students who bought fancy pennants.
Let y=number of students who bought plain pennants. 
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From the information given, you can write the following equation:
Equation#1: {{{x+y=30}}}
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From the information given, you can also write the following equation:
Equation#2: {{{4y+8x=168}}}
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Now, this is a problem of 2 equations and 2 unknowns. Any time you can develop as many equations as you have unknowns, you can solve the problem!
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Use the substitution method. First, solve Equation #1 for y:
{{{x+y=30}}}
{{{y=30-x}}}
Substitute this expression for y into Equation #2:
{{{4(30-x)+8x=168}}}
Solve for x and you are done:
{{{120-4x+8x=168}}}
{{{4x=48}}}
{{{highlight(x=12)}}}
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Good Luck,
tutor_paul@yahoo.com