Write equations with j own information:
Adult + child = 200
Adult = 200 - child
3.00 Adult + 1.50 child = 510
Combine equations:
3.00(200-child) + 1.50 child = 510
Simplify:
600 -3.00 child + 1.50 child = 510
600 - 1.50 child = 510
Subtract 600 from both sides:
-1.50 child = -90
Divide both sides by -1.50
Child = -90 / -1.50
Child = 60
There were 60 child seats sold and 140 adult seats sold.