Question

Show that if A is a subset of B then C-B is a subset of C-A

Answer 1

also an example
First take this sets
A={1,2,3}
B={1,2,3,4,5}
C={1,2,3,4,5,6,7}
Here A⊂B
C-B={6,7}
C-A={4,5,6,7}
Hence C-B is ⊂ C-A

definition od contrapositive
By definition, the reverse of an implication means the same as the original implication itself. Each implication implies its contrapositive, even intuitionistically. In classical logic, an implication is logically equivalent to its contrapositive, and, moreover, its inverse is logically equivalent to its converse.