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121 and 122
She must have made a mistake. To get a sum of 243, she should have opened the book at pages 121 and 122, but those two pages are the opposite sides of a single sheet of paper in the book, so it's impossible to "open" a book right there!
Silvia good comment- but our logic assumes the numbering is odd first, you could start with the numbering sequence 1-2, 21-22, 121-122.