Answer:
24 days.
Explanation:
First lets look at the facts.
400 ducks can eat a bunch of feed in 30 days.
That means that 200 ducks would take 60 days.
100 ducks would take 120 days.
Now, we must multiply 100 by 5 to get 500, the target amount of ducks, and divide 120 by 5 to get 24. So, 500 ducks can eat a reservoir of feed in 24 days.