The box plot has 3 parts, the 25th percentile. 50th percentile and 75th percentile.
25th percentile is the left side of the box
50th percentile is the centerline of the box
and
75th percentile is the right side of the box
Since 60 is located at the left side of the box which is the 25th percentile, therefore, the items below 60 is 25% of the data set.
