Well, we know enough information to make some equations and solve them.
Things we know:
1) 52 legs total
2) Ducks have 2 legs
3) Cows have 4 legs
4) No other legs are being counted
5) 4 more cows than ducks
We have 2 unknowns: # of ducks, we'll call that D, and # of cows,...
I'd need to see the exact question to be sure, but I think the explanation below should help.
Mode should be the same. Whichever # is repeated most often. If you have multiple repetitions of an unknown, it could end up being your mode.
Example A: 2, 4, 5, 3, 4, 6, x, 4, 2 =>...