Let x = adult tickets
Let y = children's tickets
x + y = 800
8x + 4y = 4,400
From the first eq, x = 800 - y, plug this in to the second eq
8(800-y) + 4y = 4,400
6400 - 8y + 4y = 4,400
2000 = 4y
500 = y
x = 800 - 500
x = 300
300 adult tickets were sold.