Three-eights of the 32 wild ducks feeding in the lake were wood ducks the rest were mallards how many mallards were in lake?

If three-eighths of the 32 wild ducks were wood ducks, that means there were ( \frac{3}{8} \times 32 = 12 ) wood ducks. The remaining ducks must be mallards, so the number of mallards in the lake is ( 32 - 12 = 20 ). Therefore, there were 20 mallards in the lake.