Question: A box of 15 cookies costs $9.
What is the cost of 1 cookie?
1. **State the problem:** We need to find the cost of one cookie if 15 cookies cost 9.
2. **Formula:** The cost per cookie is given by dividing the total cost by the number of cookies:
$$\text{Cost per cookie} = \frac{\text{Total cost}}{\text{Number of cookies}}$$
3. **Substitute the values:**
$$\text{Cost per cookie} = \frac{9}{15}$$
4. **Simplify the fraction:**
$$\frac{9}{15} = \frac{\cancel{3} \times 3}{\cancel{3} \times 5} = \frac{3}{5}$$
5. **Interpretation:** The cost of one cookie is $\frac{3}{5}$ or 0.6.
**Final answer:** The cost of one cookie is $0.6$.