Eliza’s backpack weighs 18 and 7 eighths pounds with her math book in it. Without her math book, her backpack weighs 14 and 7 ninths pounds. How much does Eliza’s math book weigh?

Subtract the backpack’s weight without the math book from its weight with the book: 18 7/8 − 14 7/9. The whole-number difference is 4, and 7/8 − 7/9 = 63/72 − 56/72 = 7/72. So the math book weighs 4 7/72 pounds, choice D.

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