Question 972890
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S=number of student tickets; P=number of parent tickets
S+P=300
P=300-S
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$3S+$5P=$1200 Substitute for P
$3S+$5(300-S)=$1200
$3S+$1500-$5S=$1200 
-$2S=-$300
S=150 ANSWER 1: There were 150 student tickets sold.
P=300-S=300-150=150 ANSWER 2: 150 parent tickets were sold.
Fractionally, 1/2 of the tickets were student tickets and 1/2 were parent tickets.
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CHECK:
$3S+$5P=$1200
$3(150)+$5(150)=$1200
$450+$750=$1200
$1200=$1200