Concept:
Intersection means common elements.
\[
5N=\{5,10,15,20,25,\dots\}
\]
\[
3N=\{3,6,9,12,15,18,21,\dots\}
\]
Step 1: Find common multiples.
Common elements are:
\[
15,30,45,60,\dots
\]
These are exactly the multiples of:
\[
LCM(5,3)=15
\]
So:
\[
5N\cap 3N=15N
\]
Thus, the required answer is:
\[
\boxed{15N}
\]