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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.
This is a small thing, but when you the starting code, shouldn't d be the days, not the cost?
So instead of rental_car_cost(d), put rental_car_time(d).
Thanks!