Step by step
The question
Question: on a histogram, the class 50 to 80 has frequency density 0.4: find its frequency.
class 50-80, frequency density 0.4
1 Class width
The width of the class is the end minus the start: 30.
width = 80 − 50 = 30
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: 12.
It is the area of the bar.
frequency = 0.4 × 30 = 12
Careful!
The height is not the frequency.
The area of the bar is.
Careful: not frequency = 0.4 ✗
3 Check
Let's check: 12 divided by 30 gives back the density, 0.4.
So it is right!
12 ÷ 30 = 0.4
Let's look back.
First: class width.
Then: frequency.
Last: check.
Answer
Careful: not frequency = 0.4 ✗