1. **Problem statement:** We are given a histogram showing frequency densities for different mass intervals of honey produced by beehives. We know 600 beehives produced between 16 kg and 18 kg, and we need to find the frequency density value $x$ for the interval 15 to 18 kg. 2. **Formula and concept:** Frequency density is defined as $$\text{Frequency density} = \frac{\text{Frequency}}{\text{Class width}}.$$ 3. **Given data:** - Frequency for 16 to 18 kg = 600 beehives - Class width for 15 to 18 kg = $18 - 15 = 3$ kg - Frequency density for 15 to 18 kg = $x$ (unknown) 4. **Important note:** The frequency given (600) is for 16 to 18 kg, which is a subset of the 15 to 18 kg interval. However, the histogram bar covers 15 to 18 kg, so the frequency for the entire 15 to 18 kg interval is unknown but must be at least 600. 5. **Assumption:** Since the histogram bar covers 15 to 18 kg, and the problem states 600 beehives produced between 16 and 18 kg, we assume the frequency for 15 to 18 kg is 600 (or the problem implies this). 6. **Calculate $x$:** Using the formula, $$x = \frac{\text{Frequency}}{\text{Class width}} = \frac{600}{3} = 200.$$ 7. **Answer:** The frequency density $x$ is 200.0 (to 1 decimal place). This means the frequency density on the y-axis for the 15 to 18 kg interval is $\boxed{200.0}$.