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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I would suggest to print some data that would help you make sure what the issue is. For example the groups and prices.
Might be a minor error in the formulas or a rounding error in the answer.
Even when marked as spoiler, kata's general discussion is not the place to review functional code, you should ask in your solution instead.