But that's exactly what a best practive is — chosing the best-suited answer from a number of different ones.
In my real code I would like to have this one, not any other one. Thus, this is the best practice.

(And yes, googling for the formula instead of taking a straightforward looping path is a good habit to have, IMO.)

Fixed.
I should be ashamed, taking in account I've been pointed on the same issue 3 months ago.

Thanks for your feedback.
I guess the tests are already fixed.

Great, more clever than original one I've suggested!

Perharps each_with_index {|method, i| instead of each_index {|i| and self*10+i instead of (self.to_s+i.to_s).to_i can make it even more efficient.

Pretty destructive, I must aassume.