we know the price for a child is $1.50, so if we had "c" children, the total price for the children is 1.50 * c, or 1.5c.
likewise, the price for an adult is $4, if we had "a" adults, the total price for all adults is 4*a, or 4a.
we also know that day 2200 folks went in, so, whatever "c" and "a" are, we know that c + a = 2200, and the admission collected for all those is 5050, namely 1.5c + 4a = 5050.