Me with number 4 it isn't really a big question. : /

Range is the difference between the highest and lowest values. So 20.9 - 15.8= 5.1.

Mean is the average of all the numbers. So you add them all together and then divide by how many numbers there are.
16.3 + 16.8 + 20.9 + 15.8 + 15.8 = 85.6  Now divide by 5.

17.12 is your Mean.

(Also you got mode and median wrong. Mode is which number gets repeated the most. In this case it is 15.8. Median is the middle number when the numbers are arranged from smallest to biggest. So in this case it is 16.3.)

Hope this helped you! :)