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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.
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Massive help, thanks for that.
Let's try to rephrase a bit:
Given
find the year, Y, in which the father will be (if Y is in the future) or was (if Y is in the past) twice the age of his son.
Return the difference between Y and the current year.