In Slang language, XD stands for a laughing face. (See it sideways).
From your computer, You can Type two types of Xs. A capital (X) and a Small (x).
Also, You can type Two types of Ds, A Capital (D) and a Small (d).
How Many Types of Signs can you make with the x-d combination?
Note that order of the letters matters.
The Combinations are counted, Whether they make sense, or not.
This problem is 100% Original