I believe it is 49 because it is the median of the upper half of numbers. For example, 39 is the median of the whole group of numbers therefore it is in between the inner and outer quartiles on the box in the box and whisker plot. Because 39 is the median of all the numbers, the median of the half of numbers bigger the 39 is 49, making it the outer quartile (which is the line that ends the box on the right side).
What is the third quartile of the data displayed in the box-and-whisker plot? Box-and-whisker plot ranging from 34 to 58 with ticks at increments of one half. Plot defined by points at 35, 37, 41, 49, 55.