To determine how many fish were between 30 cm and 40 cm long, we need to calculate the area of the bar in the histogram representing the range of lengths from 30 cm to 40 cm. The area is calculated as:

$\text{Area} = \text{Frequency density} \times \text{Class width}.$

From the histogram:

• Frequency density for the 30–40 cm interval is 1.
• Class width for this interval is $40 - 30 = 10$ cm.

$\text{Area} = 1 \times 10 = 10.$

The total number of fish in this interval is 10.

Would you like further clarification or a deeper explanation?

Here are five related questions to expand your understanding:

1. How do we calculate the total number of fish for the entire histogram?
2. What is the relationship between frequency density and total frequency?
3. How would the calculation change if the frequency density were not uniform?
4. How can histograms help interpret real-world data in general?
5. Why is it important to calculate the class width when analyzing histograms?

Tip: Always check the frequency density carefully, as it determines the height of the bars in a histogram!