Have you tested any of your claims? https://go.dev/play/p/596Wj16Phzj

Apparently, this can be done in O(n²) at best. Correct?

Best solution out there! Very elegant!

I'll try to figure out anyway, but I agree with you.

Could you please explain the logic behind the choice of 0.0002?

So elegant and simple. Amazing!

Equivalent to iter_s = iter(s); zip(iter_s, iter_s) or zip(s[::2], s[1::2])

In Python, the following generates a timeout:

def min_remove(A):
    pass

Given that, I don't see how one could solve this kata in the allotted time.

This could be improved: right now, you are calling b.lower() for every letter in a, and you are calling a.lower() for every letter in b.

Needs a test case for array of length 1

First, this problem is about keeping track of the different bank notes (25, 50, 100) to determine whether you can give change. Also, note that right now you are adding (25 - change) for every transaction, but every transaction should add a net amount of 25 to the cash.