the class mark of continous frequency distribution is 12,14,16,18....,then find the class interval corresponding to the class mark 16 and 22

First substract 12 from 14 and divide the resultant number by 2, the substract it to 12 and add it to 12......... therefore, class interval is 11-12, do the same to all other class marks then you will get class intevals 13-15, 15-17,17-19 etc.