Of the 50 entries in a costume contest, 31 entrants were dressed as heroes and 20 entrants were wearing a mask. if 9 entrants were dressed as heroes wearing a mask, how many entrants were neither dressed as heroes nor wearing a mask

8

Step-by-step explanation:

Given that Of the 50 entries in a costume contest, 31 entrants were dressed as heroes and 20 entrants were wearing a mask.  9 entrants were dressed as heroes wearing a mask.

Let A = heroes and B = wearing mask

n(U) = 50

n(A)=31 and n(B) = 20 and n(AB) =9

Hence n(AUB) =

Persons who were neither dressed as heroes nor wearing a mask

step-by-step explanation:

you can add the two numbers and divide the result by 2. or you can find the gap between the two numbers and add half of this to the smaller number.