A jj question

Could anyone kindly help me with it?

523. 28是perfect number那题,问所有因子倒数的和是多少
  2
            

May I ask how to get this answer?

Many thanks indeed!

28所有因子是1，2，4，7，14，28。

把这些数的倒数加起来，1，14/28,7/28,4/28,2/28,1/28，答案是2。

Thank you!