7.5a+3c=663
a+c=113
Write a literal equation for c using the second equation.
c=-a+113
Substitute the value of c into the first equation
7.5a+3(-a+113)= 663
7.5a-3a+339=663
4.5a+339=663
4.5a=324
a=72
Final answer: 72 adult tickets sold
Check by finding the value of c and plugging both values in
(72)+c=113
c=41
7.5(72)+3(41)
663
True
