Whoops...now I made an error...!!!!
[Trying to do math in my head....never a good idea !!! ]
-72A = - 216 divide both sides by -72
A = 3 = [$3 for adult tickets]
Putting this back into 23 - 6A = 23 - 6(3) = 23 - 18 = $5 for child's ticket
But...you had the right idea...that was my stupid error...!!!