You are missing a few ASCII characters with this replace (there are 6 characters between 'Z' and 'a')

Powersell - Some test case is broken - I verified all test cases (logging them and comparing the values) and all falses are indeed false, yet one of them expects true

Since I can't see your solution, I'm going to guess with most common things:

1. Limit your loops, eg. if the current sum is already bigger than desired stop iterating, don't start from 1 or 0 but from previous digit
2. Take anything that is not necessary outside the loops
3. Maybe try rearanging the loops (here point 1 might make a big difference)

C# Test error:

Expect empy string for null
Expected: null
But was: <string.Empty>

(C# 7.3) One of the tests on Attempt throws an error:

tests/Fixture.cs(37,17): warning CS0219: The variable 'Tests' is assigned but its value is never used