Gwendolyn just brought a book from the library, having 20 paragraphs per page, and since the book has 50 pages, the number of paragraphs in the book is 50*20=<<50*20=1000>>1000 paragraphs.
If each paragraph has 10 sentences, the book's number of sentences is 1000*10=<<1000*10=10000>>10000 sentences.
Since Gwendolyn can read 200 sentences of a book in 1 hour, she will read 10000 sentences in 10000/200=<<10000/200=50>>50 hours.
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