Cookies For Me

Cookie Monster's mum bakes cookies in batches of 15 for the entire family.
Cookie Monster eats 9 cookies every time, but only if there are at least 9 left.
Cookie Monster's younger sister eats cookies 1 at a time, after Cookie monster is done with them.

After a day of baking, Cookie Monster's mum sees that there are no cookies left. Given that the younger sister ate at least 1 cookie, what is the minimum possible number of cookies that she ate?

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