Since there are a adults and c children
Since there are 187 tickets sold
Then the number of adults and children is 187
Add a and c, then equate the sum by 187

Since the price of each adult's ticket is $25
Since the price of each child ticket is $13
Since the total revenue is $3223
Then multiply a by 25, c by 13, then add the products and equate the sum by $3223

Now, we have a system of equations to solve it
Multiply equation (1) by -13 to make c equal in values and opposite in signs to eliminate it

Add equations (2) and (3)

Divide both sides by 12

Substitute a by 66 in equation (1) to find c

Subtract 66 from both sides

The answers are:
a) The system of equations is

b) There were 66 adult tickets sold
c) There were 121 children's tickets sold