Answer: We have many options:
1) 15 artists, with 2 paintings per artist.
2) 10 artists, with 3 paintings per artist.
3) 6 artists, with 5 paintings per artist.
4) 5 artists, with 6 paintings per artist.
5) 3 artists, with 10 paintings per artist.
6) 2 artists, with 15 paintings per artist.
by different artists, then the number of artists must be more than 1.
each artist shows more than 1 painting: >1 painting/artist
(30 oil paintings)/(2 paintings/artist)=15 artists
(30 oil paintings)/(3 paintings/artist)=10 artists
(30 oil paintings)/(5 paintings/artist)=6 artists
(30 oil paintings)/(6 paintings/artist)=5 artists
(30 oil paintings)/(10 paintings/artist)=3 artists
(30 oil paintings)/(15 paintings/artist)=2 artists
