Step by step
The question
Question: on a histogram, the class 10 to 15 has frequency density 4: find its frequency.
class 10-15, frequency density 4
1 Class width
The width of the class is the end minus the start: 5.
width = 15 − 10 = 5
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 = 4 × 5 = 20
Careful!
The height is not the frequency.
The area of the bar is.
Careful: not frequency = 4 ✗
3 Check
Let's check: 20 divided by 5 gives back the density, 4.
So it is right!
20 ÷ 5 = 4
Let's look back.
First: class width.
Then: frequency.
Last: check.
Answer
Careful: not frequency = 4 ✗