Answer: 4 tickets for children under 12.
Explanation:
A is the number of adults, S is the number of students and C is the number of children.
We have the equations:
1) A + S + C = 24.
2) A = 2*C
3) A*$15 + S*$10 + C*$5 = $260
So we have 3 equations and 3 variables.
First, we can replace A in both of the other equations.
1) 2*C + S + C = 24
S + 3*C = 24
and
3) (2*C)*$15 + S*$10 + C*$5 = $260
C*($30 + $5) + S*$10 = $260
C*$35 + S*$10 = $260
Now, we should isolate S in the first equation, and get:
1) S = 24 - 3*C
now we can replace it on the other equation:
3) C*$35 + S*$10 = $260
C*$35 + (24 - 3*C)*$10 = $260
C*($35 - $30) + $240 = $260
C*$5 = $20
C = 20/5 = 4
So we have 4 children.