c = number of children
s = number of students
a = number of adults
since the theater has a capacity of 189, then c + s + a = 189.
there are half as many "a" as "c", so then a = c/2 or 2a = c.
so the cost for all children since the price for one is 5 bucks, the total will be 5*c or 5c.
likewise, the total cost for students is 7*s or 7s.
and for adults likewise is 12*a or 12a.
we know all ticket sales was 1372, so then 5c + 7s + 12a = 1372.
now, let's use the elimination method on that system of equations of two variables, let's multiply hmmmm say the first equation by -7, to eliminate the "s".