Let P be the number of pages Cora has to read on Thursday.
If she finishes the book by the end of Friday, she will have read 23 + 38 + 61 + P + 2P = 158 pages.
The pages she has left to read by Thursday are P + 2P = 3P = 158 - 23 - 38 - 61 = pages.
Thus, by Thursday she has 3P = 36 pages left to read.
Therefore, to finish the book on Friday, Cora has to read P = 36 / 3 = <<36/3=12>>12 pages on Thursday.
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