The description contains the following line:

Note: The following methods (among many others) have been disabled:

Note the "(among many others)" part. If I was to post the whole list of disabled operators and given the nature of Ruby, this would kind of spoil the expected solution.

The kata is labeled with "Fundamentals" though, so there's a hint there :)

This is intended, those operators have been disabled on purpose.

I am also impressed at the variety of solutions provided for this problem.

I just updated the restricted methods for any newcomer that stumbles upon this problem... I want to see if they can still solve this with another wild idea.

Nice workaround! Thank you for your help.

Looks like the environment where katas are being executed is not entirely sandboxed and deleting Fixnum#+ is messing with something...

There seems to be an issue with this kata, it could be completed before but there's something weird happening now. Wonder if jhoffner could help out?

Test cases are good, your combinations are not properly ordered. Read the description again.

True, but your comment "+1 for obfuscation" on another answer brought me here so I figured I would add some myself :)
Thank you for posting this, even though I spent some time "Submitting" answers and looking for some kind of pattern I enjoyed this kata!

These*

This kind of katas are awesome! Good job posting this.

Only one test? Easy to hardcode the answer.