The given arithmetic progression is 13,16,19,22,... When 8 is added to each term of the above arithmetic progression, a new sequence is formed as 21,24,27,29,... Thus the new sequence is also an A.P. with the common difference again as 3. When 8 is subtracted from the A.P. 13,16,19,22,... we have the new A.P. as 5,8,11,14,... with the common difference as 3 again. Thus the common difference remains unchanged even though each term of the A.P. gets added or subtracted by a fixed number such as 8 as this case.