Let x be the number of child tickets sold
Let y be the number of adult tickets sold
Amount received for the child tickets: 5.90x
Amount received for the adult tickets: 9.40y
Total sales of 1001.30:

127 tickets were sold:

Use the two equations above as a system of equations, to solve it:
1. Solve y in the second equation.

2. Use the value you get in step 1 to substitute y in the first equation:

3. Solve x:

The value of x is 55
Then, the number of child tickets is 55