Look at the following table below.
Class IntervalClass mark0−6a6−12b12−181518−2121
The missing class marks a and b are respectively ____.
3, 9
Class mark for a class interval is given by
Class mark=Upper class limit+Lower class limit2
For the 1st class interval , a=0+62=62=3.
For the 2nd class interval, b=6+122=182=9.