1. **State the problem:** Sandy bought three items: a calculator, a thumb drive, and a phone case. The ratio of their costs is 7 : 13 : 4, and the total cost is 72. 2. **Formula and approach:** Let the common multiplier be $x$. Then the costs are: $$\text{Calculator} = 7x, \quad \text{Thumb drive} = 13x, \quad \text{Phone case} = 4x$$ The total cost is the sum of these: $$7x + 13x + 4x = 72$$ 3. **Simplify the equation:** $$7x + 13x + 4x = 24x = 72$$ 4. **Solve for $x$:** $$x = \frac{72}{24}$$ Show cancelation: $$x = \frac{\cancel{72}}{\cancel{24}} = 3$$ 5. **Find the cost of the phone case:** $$\text{Phone case} = 4x = 4 \times 3 = 12$$ **Final answer:** The phone case cost 12.