Forty students watched films A, B and C over a week. Each student watched either only one film or all three. Thirteen students watched film A, sixteen students watched film B and nineteen students watched film C. How many students watched all three films?
Since students watch either 1 or all 3:
let a = students who watched only A,
b = students who watched only B,
c = students who watched only C,
n = students who watched A, B and C.
Given data implies
a + n = 16 ……….
b + n = 13 ……….
c + n = 19 ……….
and since total strength of class is 40,
a+b+c+n = 40 ……….
solving for n (  +  +  –  )
2n = 8