Best practice for sure!

For some reason I didn't understand this. In the end I did it through laziness.

What do the question marks in front of letters mean? (?a..?z)

I think this is the best solution, since everything is done here in one loop. Genius!

You can also use the group_by method

The road will be mastered by the walking one, so don't worry much.

The best ruby-way

The sum method in this case is more concise and works faster than inject.

Use single quotes unless string interpolation is required.

Yes. But another question: is it best practice to use such a non-concise code? You must remember the order of operations. In my opinion, it is really hard to read. I would use parentheses instead.