Step by step
The question
Question: on a histogram, the class 40 to 100 has frequency density 0.25: find its frequency.
class 40-100, frequency density 0.25
1 Class width
The width of the class is the end minus the start: 60.
width = 100 − 40 = 60
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: 15.
It is the area of the bar.
frequency = 0.25 × 60 = 15
Careful!
The height is not the frequency.
The area of the bar is.
Careful: not frequency = 0.25 ✗
3 Check
Let's check: 15 divided by 60 gives back the density, 0.25.
So it is right!
15 ÷ 60 = 0.25
Let's look back.
First: class width.
Then: frequency.
Last: check.
Answer
Careful: not frequency = 0.25 ✗