1. **State the problem:** Tristan took his computer to a repair store. The technician worked for 4.75 hours and charged $50 for parts. The total bill was $643.75. We need to find the cost of labor per hour. 2. **Set up the equation:** Let $x$ be the cost of labor per hour. The total cost is the sum of labor cost and parts cost: $$\text{Total} = \text{Labor cost} + \text{Parts cost}$$ $$643.75 = 4.75x + 50$$ 3. **Isolate the variable $x$:** Subtract 50 from both sides: $$643.75 - 50 = 4.75x$$ $$593.75 = 4.75x$$ 4. **Divide both sides by 4.75 to solve for $x$:** $$x = \frac{593.75}{4.75}$$ Show cancellation: $$x = \frac{\cancel{593.75}}{\cancel{4.75}}$$ 5. **Calculate the division:** $$x = 125$$ 6. **Answer:** The cost of labor per hour is **125** per hour.