Given,
The block post of the data is shown in the question.
Required
The difference between the median and third quartile of the data.
As known that the block post represent the minimum, maximum, first quartile, median and third quartile of the data.
From the given block post, it is clearly seen that the median of the data is 17.
The third quartile of the data is 20.
The difference in median and third quartile is,

Hence, the difference is 3.