Please explain this question

Let X = {1, 2, 3, 4, 5}. The number of different ordered pairs (Y, Z) that can be formed such that Y⊆X, Z⊆X and Y∩Z is empty, is

let an element $a\in X$
therefore following are the cases:
$$\begin{gathered} 1. a\in Y and a\in Z \\ 2. a\in Y and a\notin Z \\ 3. a\notin Y and a\in Z \\ 4. a\notin Y and a\notin Z \end{gathered}$$$$\begin{gathered} 1.a\in Y and a\in Z \\ 2.a\notin Y and a\in Z \\ 3.a\in Y and a\notin Z \\ 4.a\notin Y and a\notin Z \end{gathered}$$
since the given condition is $Y\cap Z=\varphi$
therefore for any element of X , there are 3 ways.
either it is an element of Y or it is an element of Z or it is neither the element of Y nor Z
thus the number of ordered pairs = 3*3*3*3*3 = $3^5$

hope this helps you