In Python, you may use decimal module.

I don't understand what you mean. You have to return all pairs that work and only these pairs. Moreover it seems you understood since you passed.

Not an issue with the kata.

Generally this comes with experience, familiarity of libraries and frameworks, and repetition. You will also get there if you follow these simple steps:

1. Solve kata
2. Read solutions of others
3. Forget how you solved your kata
4. Go to pt 1, repeat 5 times
5. Change language
6. Go to 1, 3 times

For me, the most helpful step is pt 2. Smart people are invaluable source of knowledge.

All of the ?s in an expression will represent the same digit (0-9), and it won't be one of the other given digits in the expression.