Principal ( P = 5000 ), rate ( r = 8% ).

Interest for one year ( = \dfrac{5000\times 8\times 1}{100} = 400 ).

Interest at the end of each year (simple interest is calculated on the original principal each year): [ 400, 400, 400, \ldots ] (This is a constant sequence, which is a special A.P. with ( d = 0 ).)

Interest after 20 years ( = 20\times 400 = 8000 ).

Answer: Interests form an A.P. (with ( d=0 )); interest after 20 years = Rs. 8000