Step by step
The question
Question: on a histogram, the class 20 to 40 has frequency density 1.5: find its frequency.
class 20-40, frequency density 1.5
1 Class width
The width of the class is the end minus the start: 20.
width = 40 − 20 = 20
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: 30.
It is the area of the bar.
frequency = 1.5 × 20 = 30
Careful!
The height is not the frequency.
The area of the bar is.
Careful: not frequency = 1.5 ✗
3 Check
Let's check: 30 divided by 20 gives back the density, 1.5.
So it is right!
30 ÷ 20 = 1.5
Let's look back.
First: class width.
Then: frequency.
Last: check.
Answer
Careful: not frequency = 1.5 ✗