Question 1005722
At the city museum, child admission is $5.10
and adult admission is $9.40
On Monday, 139 tickets were sold for a total sales of 
$1065.80
How many adult tickets were sold that day? 
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Equations:
Quantity:: c + a = 139 tickets
Value:::  5.1c + 9.4a = 1065.8 dollars
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Modify for elimination::

51c + 51a = 51*139
51c + 94a = 10658
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Subtract and solve for "a"::
43a = 3569
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a = 83 (# of adult tickets sold)
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Cheers,
Stan H.
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