1. **State the problem:** We know a pen costs 10 and a pencil costs 5. Two pens are bought, and the total cost is 45. We need to find how many pencils were bought. 2. **Define variables:** Let $x$ be the number of pencils bought. 3. **Write the equation:** The total cost is the cost of 2 pens plus the cost of $x$ pencils: $$2 \times 10 + 5x = 45$$ 4. **Simplify the equation:** $$20 + 5x = 45$$ 5. **Solve for $x$:** $$5x = 45 - 20$$ $$5x = 25$$ $$x = \frac{25}{5} = 5$$ 6. **Answer:** 5 pencils were bought.