If c=no. of cows and d=no. of ducks,
4c+2d=148 feet, 2c+d=74.
There are an even number of ducks.
If d=2, c=36; d=4, c=35; d=6, c=34; etc.
Since d=74-2c=2(37-c), so the number of cows must be less than 37. If c=2 at minimum, the maximum number of ducks is 70.
All possible solutions are in the table below.
COWS DUCKS COWS DUCKS
2 70 11 52
3 68 12 50
4 66 13 48
5 64 14 46
6 62 15 44
7 60 16 42
8 58 17 40
9 56 18 38
10 54 19 36
COWS DUCKS COWS DUCKS
20 34 29 16
21 32 30 14
22 30 31 12
23 28 32 10
24 26 33 8
25 24 34 6
26 22 35 4
27 20 36 2
28 18