it's the area of the columns that give you a value rather than the height like in a bar chart.
frequency = frequency density * class width
this can be easily rearranged to suit what you need to find out in an exam
In an exam, you will be given an incomplete table and an incomplete graph. Youwill need to find the frequency from a column but before this, you will need to work out the frequency density by dividing the frequency (from the table) by the class width so you know the value in which you multiply by the class width.
Just remember the triangle:
FD * CW