The intersection of two sets is the set composed by the elements that the two sets have in common.
So, we have
![X\cap Y = \{100,\ 1000\}](/tpl/images/0364/0868/a185b.png)
In this particular case, since Y is a subset of X, we have
![X\cap Y=Y](/tpl/images/0364/0868/f9c01.png)
In fact, all elements of Y are elements of X as well, so in particular they are common elements.