Question 849684
sum of arithmetic series:

Sn = (a1 + an) * n / 2

= (9+an)*41/2 = 9989

solving for An we get 

An = 478.27

An = A1 + (n-1)*d  where d is the common difference

478.27 = 9 + (41-1)*d

478.27 - 9 = 40d

469.27 = 40d

d = {{{highlight(11.73)}}}