Let arrange the date properly

5/2 artists ---> make 5/2 paintings ---> 5/2 canvases ====> in 5/2 days

5/2 artists ---> make 1 paintings ---> 5/2 canvases ====> in 5/2 days/5/2 i.e. 1 day

5/2 artists ---> make 1 paintings ---> 1 canvases ====> 1 day/5/2 i.e. 2/5 day

1 artist ---> make 1 paintings ---> 1 canvases ====> 2/5 day*5/2 i.e 1 (here * will come as less artist more days)

Now we have

1 artist ---> make 1 paintings ---> on 1 canvases ====> in 1 day

Now solve the given one

1 artist ---> make 1 paintings ---> on 25 canvases ====> in `1*25`

i.e. 25 days

1 artist ---> make 25 paintings ---> on 25 canvases ====> in `25*25`

days

or

25 artist ---> make 25 paintings ---> on 25 canvases ====> in `25*25`

days/25 i.e. 25 days

**So total 25 artist required**