Kyu ratings are language-independent.

This isn't a kata issue, it is CW spazzing.

I think calling include? each time may not be very faster

Nice solution. I should have thought this!

You don't need last .to_a. map will always return an array

Ahha! You spoiled it for me!! Please mark this comment as Spoiler

After trying the next day, I could submit Final. Don't know what causes it. A cache issue may be!

Why I don't get a Final Submit button, yet I passed all test in Submit?

Nice and simple. I solved with somewhat same thing, but your's is concise and beautiful.

Why can't I do a Final Submit? My submission was oK, but can't finalize!!

I don't understand what does this mean? at least, is this feedback wrong?

Nice! It is chainable from left to right but doesn't evaluate like normal mathematical order. For example Calc.new.four.plus.four.times.two returns 16 while Calc.new.four.times.four.plus.two returns 18 which is correct.

For ruby, it is too much for 6kyu, In my humble opinion

lol! That made me grin!