Explanation:

Let the number of ducks be d

and number of cows be c

Then, total number of legs = 2d + 4c = 2$\left(d + 2c\right)$

total number of heads = c + d

Given that total number of legs are 28 more than twice the number of heads

=> 2(d + 2c) = 28 + 2(c + d)

=> d + 2c = 14 + c + d

=> 2c = 14 + c

=> c = 14

i.e., total number of cows = 14