1. **State the problem:** A bookstore owner buys 10 books for 900 when the price of each book increases by 3 Birr. We need to find the original price of one book. 2. **Define variables:** Let the original price of one book be $x$ Birr. 3. **Set up the equation:** When the price increases by 3 Birr, the new price per book is $x + 3$. 4. **Calculate total cost at increased price:** The total cost for 10 books at the increased price is $10(x + 3)$. 5. **Given total cost:** The owner pays 900 Birr for 10 books at the increased price, so: $$10(x + 3) = 900$$ 6. **Solve for $x$:** $$10x + 30 = 900$$ $$10x = 900 - 30$$ $$10x = 870$$ $$x = \frac{870}{10} = 87$$ 7. **Interpretation:** The original price of one book is 87 Birr. **Final answer:** 87 Birr (Option C)