The range is the distance between the largest and smallest numbers in a set of data (Largest number - smallest number = range)
Here we know the smallest number (20) but the largest number is at some point between 60 and 80.
To find the increment of each line in-between 60 and 80:
80-60 = 20
20/5 = 4
So the values will be 60, 64, 68, 72, 76, 80
Therefore the largest value is at the 76 mark
Now to find range by subtracting the smallest number from the largest number:
76-20 = 56
The range is 56