Wich of the following is not a factor of 80?
2 and 80
2 and 40
4 and 20
8 and 10

Factors are the 'two numbers multiplied' that give you 80. 40 and 2 give you 80, right? So does 20 and 4 & 8 and 10 Which means that the only ones that cannot be factors are 80 and 2 because those two multiplied together equal 160. 

But of course, all of these numbers individually are factors of 80, but this problem specifically asks for pairs and not the individual factors alone.

I hope you benefit from my explanation.