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Nolan has read his sister the mermaid book 3 times out of every 5 times she picks a book. So, the mermaid book has been chosen 3 out of 5 times.
Out of 20 times Nolan has read his sister a book, he has read the mermaid book 3/5 x 20 = 12 times.
Therefore, Nolan has read the mermaid book 12 times this month.