Question 693890
Let A = the cost of a child
Let B = the cost of an adult
Equation 1: {{{8A + 3B = 39}}}
Equation 2: {{{11A + 2B = 43}}}
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You can rewrite the equations to make them equal zero.
Then you can set the two equations equal to each other.
{{{8A + 3B - 39 = 11A + 2B - 43}}}
Subtract 8A from both sides
{{{3B - 39 = 3A + 2B - 43}}}
Add 39 to both sides
{{{3B = 3A + 2B - 4}}}
Subtract 2B from both sides
{{{B = 3A - 4}}}
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Now plug (3A - 4) into one of the equations for B
Equation 2: {{{11A + 2B = 43}}}
{{{11A + 2*(3A - 4) = 43}}}
Simplify
{{{11A + 6A - 8 = 43}}}
Combine like terms
{{{17A - 8 = 43}}}
Add 8 to both sides
{{{17A = 51}}}
Divide both sides by 17
{{{highlight(A = 3)}}}

Now using A = 3, solve for B
{{{B = 3A - 4}}}
{{{B = 3(3) - 4}}}
{{{B = 9 - 4}}}
{{{highlight_green(B = 5)}}}