Equivalence relationship: parking fee for small car+parking fee for medium car = total parking fee.

Solution: Suppose there are X small cars and (50-x) medium-sized cars.

4x+6 (50x) =230

4x+300-6x=230

-2x=-70

x=35

50-35 = 15 ... medium-sized cars

A: There are 35 small cars and 0/5 medium-sized cars/KLOC.