Let the number sold advance be = x
Since those at the door was 206 more than advance, therefore number sold at the door = (x + 206)
Advance cost =$6, x number sold, total =6*x =6x
Door cost = $10, (x + 206) sold, total = 10*(x +206) =10x +2060
Total cost of tickets sold = $6828
Cost of Advance + Cost at Door = 6828
6x + 10x +2060 =6828
16x = 6828 - 2060
16x = 4768
x = 4768 / 16 = 298, recall x is the number sold advance.
At the door = x + 206 = 298 + 206 = 504
Therefore 298 tickets were sold advance and 504 were sold at the door.
Hope this explains it.