1. The problem states that the ratio of blueberries to grapes is 3 to 5. 2. We know there are 15 grapes in the box. 3. Since the ratio is 3 blueberries for every 5 grapes, we can set up a proportion to find the number of blueberries: $$\frac{3}{5} = \frac{x}{15}$$ where $x$ is the number of blueberries. 4. Cross-multiply to solve for $x$: $$3 \times 15 = 5 \times x$$ 5. Simplify: $$45 = 5x$$ 6. Divide both sides by 5: $$x = \frac{45}{5} = 9$$ 7. Therefore, there are 9 blueberries in the box.