Variables
x: cost of an adult ticket, in dollars
y: cost of a child ticket, in dollars
If 1 adult ticket is sold, then x dollars are raised. In consequence, if 200 adult tickets are sold, then 200x dollars are raised.
Similarly, if 250 child tickets are sold, then 250y dollars are raised.
Combining these amounts and raising $3800, we get:
200x + 250y = 3800
