we can use formula

where
fL is listener frequency
fs is siren frequency
va is the speed of sound in air
vs is police car speed
so, we have



now, we can change it into m/s


now, we can plug it

..............Answer
To solve this, I would just find what two adult tickets and one child ticket wold be worth.
To find that, just do:
5.20 + 9.90 * 2, which makes 25.
So now you just find how many 25s are in 900, or 36.
So 36 children's tickets were sold that day.
Hope I helped :)