Step by step
The question
Question: on a histogram, the class 0 to 25 has frequency density 0.8: find its frequency.
class 0-25, frequency density 0.8
1 Class width
The width of the class is the end minus the start: 25.
width = 25 − 0 = 25
On these histograms the height is the frequency density: the frequency divided by the class width.
frequency density = frequency ÷ width
2 Frequency
So the frequency is the density times the width: 20.
It is the area of the bar.
frequency = 0.8 × 25 = 20
Careful!
The height is not the frequency.
The area of the bar is.
Careful: not frequency = 0.8 ✗
3 Check
Let's check: 20 divided by 25 gives back the density, 0.8.
So it is right!
20 ÷ 25 = 0.8
Let's look back.
First: class width.
Then: frequency.
Last: check.
Answer
Careful: not frequency = 0.8 ✗