Statement 1: If B=U−A then n(B)=n(U)−n(A) where U is universal set.  Statement 2: For any three arbitrary sets A,B,C we have  if C=A−B then n(C)=n(A)−n(B) .

If U is universal set then B=U−A=A′, for which n(B)=nA′=n(U)−n(A) But for any three arbitrary sets A,B,C we cannot have always n(C)=n(A)−n(B) if C=A−B as it is not specified here that  weather A is universal set or not. In case A is not universal set  we cannot conclude that n(C)=n(A)−n(B) . Hence, statement 1 is true but statement 2 is false.