Add every two consecutive lines in a column and divide it with the former and Print

I have a input file of almost 2000 lines. I have to do a calculation and print it in the third column.

Example of what I have to do with the Input File:

n ID1_1 n/(n+k)
k ID1_2

Input File:

 10 ID1_1
 20 ID1_2
  1 ID3_1
  9 ID3_2
 15 ID2_1
300 ID2_2

Expected Output:

 10 ID1_1 0.33
 20 ID1_2 
200 ID3_1 0.11
  9 ID3_2
 15 ID2_1 0.047
300 ID2_2

Do you have any easy way to solve it? Thanking you.