We have
x- number of child admissions
y= number of adult admissions
Therefore we have the next equation
x+y=172
5.30x+9.90y=1344
We isolate x from the first equation
x=172-y
we susbtitute in the second equation
5.30(172-y)+9.90y=1344
we simplify
911.6-5.30y+9.90=1344
4.6y=1344-911.6
4.6y=432.4
y=432.4/4.6
y=94
Then we substitute y in the equation with x isolated
x=172-94=78
ANSWER
The number of child tickets is 78