[R] Data manipulation

[mathijsdevaan]

Hi,

I have a dataset with info on individuals (B) that have been involved in
projects (A) during multiple years (C). The dataset contains three columns:
A, B, C. Example:
   A  B  C
1 1  a  1999
2 1  b  1999
3 1  c  1999
4 2  c  2001
5 2  d  2001
6 3  a  2004
7 3  b  2004

I am interested in the average tenure of all individuals for each project
(assuming that the tenure of an individual = 0 in the first project this
individual is involved in). So based on the data above:
  A  D
1 1  0
2 2  1
3 3  5

where D = average project tenure. How do I do this?

Your help is very much appreciated. Thanks!
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