Call the books A, B, C, D.

The three thirds of any book can be represented by the book letter followed by 1, 2 or 3.

Now write down the thirds of the books in a line:

A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3

From this picture, where the reading material is split into pairs representing 2/3 of a book or the final 1/3 of a book and the first third of the next, it’s easy to see how many nights it will take him to read all of the books. It will take 6 nights. The same result is obtained by dividing 4 by ⅔=4×3/2=6.

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