x= cost of 1 adult ticket
y=cost of 1 kid ticket
so 1 adult+4 childeren=x+4y=19
2 adults and 2 childern=2x+2y=17
elimination
x+4y=19
2x+2y=17
multiply firest equation by -2
-2x-8y=-38
add to first equatuion
-2x-8y=-38
2x+2y=17 +
0x-6y=21
-6y=-21
multiply -2
6y=21
divide by 6
y=21/6=7/2
subsitute
2x+2y=17
2x+2(7/2)=17
2x+14/2=17
2x+7=17
subtract 7 from both sides
2x=10
divide by 2
x=5
1 adult ticket costs £5
1 kid ticket costs £7/2 or £3 and 1/2 or £3.5
