Let U= set of individuals suffering from the skin disorder A= set of individuals exposed to the chemical C1 B= set of individual exposed to the chemical C2 Then n(u)=200,n(A)=120,n(B)=50 and n(A∩B)=30 From the Venn diagram, we have A=(A−B)∪(A∩B) ∴n(A)=n(A−B)+n(A∩B) A−B and A∩B are disjoint ∴n(A−B)=n(A)−n(A∩B) =120−30=90 Hence, 90 individuals exposed t chemical C1 but not to chemical C2