Answer:
80 
Explanation:
This is calculated as:
(Number of ducks with bands × Number of examined ducks)/ Number of ducks found with leg band
= Number of ducks with bands = 12 ducks
 Number of examined ducks = 20 ducks 
 Number of ducks found with leg band = 3 ducks
Hence:
12 × 20/3
= 240/3
= 80 ducks
Therefore, 80 ducks used the ponds