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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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The test is actually correct; I got the same "error" as well, but if you include the minute at 12:15, up to and including 13:29, it comes out to exactly 90. An easy way to figure this out is by considering 9:00 to 9:04, which is a total of 5 minutes (9:00, 9:01, 9:02, 9:03, 9:04).