Let c be the number of cows and d be the number of ducks.
c cows have 4c feet, d ducks have 2d feet. In total, c cows and d ducks have
4c + 2d = 148 feet.
Divide it by 2:
2c + d = 74.
This gives us that the number of ducks is even and cannot be 0 (because farmer Brwn has ducks). The minimum possible number of ducks is 2, so the maximum possible number of cows is 36.