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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.
loaded-icosahedron, that exercise requires another kata just to decode what it is saying xD
Either matrices or linear equations. If you want the hint clarified, look at this exercise: https://mitpress.mit.edu/sites/default/files/sicp/full-text/book/book-Z-H-11.html#%_thm_1.19
matrices
it wants undefined if none of the integers in the array sum up as pair to the sum value that was given. in that case they expect you to return undefined :)