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