Duduo evolves into Dudrio at Lv. 31.
I believe it evolves on level 31.
Level 31, Doduo evolves into Dodrio
Doduo evolves into Dodrio, starting at level 31.
In Pokémon Ruby, Gulpin evolves at Level 26 and it evolves into a Swalot.
It evolves at level 35. It evolves into Vibrava.
Level 42
25
it evolves at level 17
It evolves at level 20
Koffing evolves at Level 35 in Pokémon Ruby.
he evolves at lv 40 and he learns hyper beam at level 40
If you mean Slakoth: Slakoth evolves into Vigoroth at level 18, who evolves into Slaking at level 36.