You can
put this solution on YOUR website!Remember, if C is subset of D and D is a subset of C, then sets C and D are the same sets (draw a picture if you're unsure). In notation form, this looks like:
If

and

, then
a) So n(C)=5 also tells us that n(D)=5 (ie the number of elements in sets C and D is 5) since C=D
b) The other relation ship is that C=D