
Answer:
380 tickets were sold at the door and 440 tickets were sold before
Step-by-step explanation:
x = tickets sold before
y = tickets sold at the door
x + y = 820
x = y + 80
y + 80 + y = 820
  -80      -80
2y = 740
y = 370
820 - 380 = 440
x = 440