I was given the following question in a homework assignment, and I'm confused as to what it's asking for:

" Design a three variable circuit that outputs 1 when there are even number of 1’s. Apply Duality theorem to the resulting logical expression. What would be the behavior of the corresponding dual circuit?"

What exactly does it mean when it says " when there are even number of 1's"? I'm just a little lost on what exactly it's asking me to do

 

You have three inputs. Some of them are 0's and some of them are 1's. If the inputs are {0,1,1} how many 1's are there? Is this number even or odd?