Lets name adult price as X and child price as Y
then
1X+3Y=27 (1)
2X+5Y=48 (2)
From (1) we see that X=27-3Y
Lets replace X in (2) with this formula
2(27-3Y)+5Y=48 or 54-6Y+5Y=48 and 54-Y=48 and Y=54-48 so Y=6
From (1) X=27-3*6; X=27-18; X=9
Adult = 9€
Child = 6€