What is the third quartile of this data set?20, 21, 24, 25, 28, 29, 35, 37, 42A.35B.28C.22.5D.36

To find the third quartile (Q3) of a data set, follow these steps:

1. Arrange the data in ascending order: 20, 21, 24, 25, 28, 29, 35, 37, 42
2. Divide the data into four equal parts. The third quartile (Q3) is the value that separates the first 75% of the data from the last 25%.
3. Since there are 9 data points, the third quartile will be the value of the (3/4)*(9+1) = 7.5th position.
4. Since the position is not a whole number, we take the average of the 7th and 8th data points.
5. The 7th data point is 35 and the 8th data point is 37.
6. The average of 35 and 37 is (35+37)/2 = 36.

So, the third quartile (Q3) of this data set is 36 (Option D).