To find out how many fish were between 30 cm and 40 cm long, we need to examine the histogram. The bar for the range between 30 cm and 40 cm shows a frequency density of 1. This frequency density is for a class width of 10 cm (from 30 cm to 40 cm).

To find the number of fish in that range, we multiply the frequency density (1) by the width of the class (10 cm):

$\text{Number of fish} = \text{Frequency density} \times \text{Class width} = 1 \times 10 = 10$

So, there are 10 fish that are between 30 cm and 40 cm long.

Would you like further details or clarification on any part of the process?

Here are some related questions you might find useful:

1. How do we calculate the total number of fish based on the histogram?
2. What is the interpretation of frequency density in a histogram?
3. How do histograms represent data when the class widths are unequal?
4. What is the relationship between bar height and frequency density in a histogram?
5. How can we calculate the cumulative frequency using a histogram?

Tip: When interpreting a histogram, remember that the area of each bar represents the frequency of data within that range.