Step by step
The question
Question: on a histogram, the class 30 to 60 has frequency density 1.2: find its frequency.
class 30-60, frequency density 1.2
1 Class width
The width of the class is the end minus the start: 30.
width = 60 − 30 = 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: 36.
It is the area of the bar.
frequency = 1.2 × 30 = 36
Careful!
The height is not the frequency.
The area of the bar is.
Careful: not frequency = 1.2 ✗
3 Check
Let's check: 36 divided by 30 gives back the density, 1.2.
So it is right!
36 ÷ 30 = 1.2
Let's look back.
First: class width.
Then: frequency.
Last: check.
Answer
Careful: not frequency = 1.2 ✗