What is the third quartile of this data set 21,24,25,28,2935,37,39,42

Definitions:

The third quartile, denoted by Q3, is the median of the upper half of the data set. This means that about 75% of the numbers in the data set lie below Q3 and about 25% lie above Q3;

The upper half of a data set is the set of all values that are to the right of the median value when the data has been put into increasing order;

First, we verify data in increasing order:

21, 24, 25, 28, 29, 35, 37, 38, 42 (okay);

The median is 29;

Therefore, the upper half of the data is {35, 37, 39, 42};

Q3 = (37 + 39) / 2 = 76 / 2 = 38;