Answer: there were sold 321 adult tickets and 227 child's tickets.
Explanation:
Let x be the number of adults and y the number of childrens.
x+y=548 people because thats the whole number of sold tickets,
and
6.50*x+3.50*y=2881 dollars in total.
Let separate x from the first formula
x=548-y,
and replace it at the second equation:
6.50*(548-y)+3.50y=2881;
6.50*548-6.50y+3.50y=2881;
3562-3y=2881;
3y=681;
y=227.
So, x=548-y=548-227=321.
Hence, there were sold 321 adult tickets and 227 child's tickets.