This would actually be a fun interview question - how to emergency patch 1B+ globally distributed mobile devices. I would say at least several days for the obvious QA which needs to be done.
And you would unfortunately, not get the job.
As if that is not enough, the IOS library for some reason had implemented "hand rolled "caching with a lot of hardcoding.
To fix the issue... i made c.com homepage also serve b.js at the bottom.
There is no way in hell this would be committed and tested within qa timelines of either a.com set up or c.com setup.
It's rather complex to fully explain without giving more context on what it is but when traffic drops 40% and you are generating 5M$/day.... You do whatever to bring it back.