The walkers pay 22 for 2 adult tickets and one child ticket so we can say:
2a+c=22, The question wants to find the cost of a child ticket so solve this for a...
2a=22-c
a=(22-c)/2
Now we are told that the Goodmans by 26 for one adult and three childrens tickets so:
a+3c=26, now use (22-c)/2 in this equation for a and you get:
(22-c)/2+3c=26
(22-c+6c)/2=26
22+5c=52
5c=30
c=6
So a child's ticket costs £6.00