Hello,
Let x the cost of a child's ticket
2x is the cost of an adult's ticket
2*(2x)+3x=28
==>4x+3x=28
==>7x=28
==>x=4
2x=2*4=8
Proof:
2 adult's tickets: 2*8=16
3 child's tickets: 3*4=12
and 16+12=28.
Adult's ticket cost is 8
Child's ticket cost is 4