Let x be the number of children and let y be the number of adults.
We know that the total number of attendants was 147, then:

We also know that each child ticket cost $4 and for each adult cost $12, and the total amount collected were $1156. Then we have:

Hence we have the system:

Now we have to solve the system. To do that we solve the first equation for y:

and we plug this value into the second equation and solve for x:

Now that we have the value we can find the value of y, then:

Therefore there were 76 children and 71 adults.