EXPLANATION
Let's see the facts:
Museum visitors 531 469 573 206 374 421 505 489 702
Ordering the data to find minumun, maximun and quartiles:
206 374 421 469 489 505 531 573 702
The minimun of this data set is 206.
The first quartile will be the median of the first half of the data set:
206 374 421 469 ---> (374+421)/2 = 397.5
Now we need the median. If there are even number of data, the median is the average of the middle two data:
489
The third quartile is the median of the second half of the data set:
505, 531 , 573, 702 --> (531+573)/2= 552
The maximun is the greatest value in the data set:
702
Finally, drawing the box-and-whisker plot: