Step by step
The question
Question: on a histogram, the class 15 to 20 has frequency density 2.4: find its frequency.
class 15-20, frequency density 2.4
1 Class width
The width of the class is the end minus the start: 5.
width = 20 − 15 = 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: 12.
It is the area of the bar.
frequency = 2.4 × 5 = 12
Careful!
The height is not the frequency.
The area of the bar is.
Careful: not frequency = 2.4 ✗
3 Check
Let's check: 12 divided by 5 gives back the density, 2.4.
So it is right!
12 ÷ 5 = 2.4
Let's look back.
First: class width.
Then: frequency.
Last: check.
Answer
Careful: not frequency = 2.4 ✗