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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.
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Chrono79 said it all.
It seems you're returning a float instead of a double. See the type of your function in the initial code.
I'm getting similar issues with this kata on the last expected test in C++. The test is expeting 12.74 and my result is 12.74 but still marks it as incorrect.