How many ways can you choose three letters from the letters
of the word uncopyrightable?

 http://discrete.openmathbooks.org/dmoi2/sec_counting-combperm.html

https://dr282zn36sxxg.cloudfront.net/datastreams/f-d%3A0f6194861caf415488f3b1e328fd7cd629d328bce6366a2beb7c79f2%2BQUIZ%2BQUIZ.1

Explanation:  https://www.chegg.com/homework-help/questions-and-answers/6-many-ways-arrange-15-letters-word-uncopyrightable-b-many-ways-arrange-3-letters-word-add-q44031586

https://math.stackexchange.com/questions/734771/how-many-ways-can-six-of-the-letters-of-the-word-algorithm-be-selected-and-writt