x = number of tickets sold in advance, and thus cheaper.
x + 206 = number of tickets sold at the door.
now, the ones sold at the door, are more expensive and is more than the early sold tickets, as you know is x + 206.
now, if the early ones cost each one 6 bucks, then the cost for all of them is 6*x, or 6x.
the ones sold at the door are 10 bucks each, and therefore their total cost is 10(x + 206).
now, we know sales in total for both was 6828, therefore,
[tex]\bf \stackrel{in~advance}{6x}~~+\stackrel{at~the~door}{10(x+206)}=6828\implies 6x+10x+2060=6828
\\\\\\
16x=4768\implies x=\cfrac{4768}{16}[/tex]
and surely you'd know what that is.
how many were sold at the door? well, x + 206.
