Answer:
84 child tickets were sold
Explanation:
Had they all been adult tickets, revenue would have been ...
$9.80 × 147 = $1440.60
It was less by ...
$1440.60 -1054.20 = $386.40
The child's ticket costs less by ...
$9.80 -5.20 = $4.60
so there must have been $386.40/$4.60 = 84 child tickets sold.
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Replacing an adult ticket with a child ticket reduces the revenue by $4.60 without changing the number of tickets sold.
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You can let c represent the number of child tickets sold. Then (147 -c) is the number of adult tickets sold, and total revenue is ...
5.20c + 9.80(147 -c) = 1054.20
-4.60c +1440.60 = 1054.20 . . . . simplify
-4.60c = -386.40 . . . . . . . . . . . . . subtract 1440.60
c = 386.40/4.60 = 84