In percent bar method, there are two things to be considered.
Part and the whole.
part is considered as the one of the total number of divisions, whereas, the whole is the summation of all the parts.
Now, in the figure, where the missing percent is mentioned, is the first part,
Part = 1
Total = 5
Now as bar represent 100%.
Since, there are 5 partitions, We divide 100 with 5 to determine amount of percent represent by each bar or partition.
Each bar = 100/5 =20%
Since we have to calculate the first bar,
So First bar = 20%,
Note that if we have to calculate two bars, it will be 20 +20 = 40%
But since we have to calculate only first partition so answer is 20%
1/5 of 100 is 20.
You find this by dividing 100 by 5 to get 20, then you subtract 20 from 100.