Make a system of equations to solve this. We will make x be the #children and y the # of adults:
x+y=2500
5.5x+11.5y=19750
Now we change the first equation to isolate one of the variable:
x=2500-y
now that we know what x kinda equals we can put it into the other equation and solve:
5.5(2500-y)+11.5y=19750
13750-5.5y+11.5y=19750
6y=6000
y=1000
Now we can plug our new value for y into the first original equation:
x+1000=2500
x=1500
So we know there were 1500 children and 1000 adults.