Question 787506
x, y, z, count of child tickets, adult tickets, senior tickets.

x=3+y, and y-1=z.
Accounting for cost, 10x+15y+13z=131.


You could solve the two first equations for y, and equate those expressions, and use one of them to substitute for y in the cost equation.  You then have any two equations in x and z.  


Tell if you need further help than this.