Dear Stata Users,

Please, help me with the following issue. I need to calculate reserves based on the following idea. Below is a firm (gvkey) and year (fyear) observations of exports (exp). What I want to calculate is as follows. For example, for firm (gvkey) 001013 reserves are expected to be as follow:
Each year the reserve is calculated as an “exp” multiplied by:
Yr1: 10/15
Yr2: 6/15
Yr3: 3/15
Yr4: 1/15
Yr5: 0
Example:
gvkey fyear exp reserve
1013 2003 108.6 108.6*10/15
1013 2004 59.1 108.6*6/15 + 59.1*10/15
1013 2005 . 108.6*3/15 + 59.1*6/15
1013 2006 72 108.6*1/15 + 59.1*3/15 + 72*10/15
1013 2007 . 59.1*1/15 + 72*6/15
1013 2008 83.5 72*3/15+83.5*10/15
1013 2009 65.3 72*1/15+83.5*6/15 + 65.3*10/15
1013 2010 69.7 83.5*3/15 + 65.3*6/15+69.7*10/15


Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input str6 gvkey double(fyear exp)
"001013" 2003   108.6
"001013" 2004    59.1
"001013" 2005       .
"001013" 2006      72
"001013" 2007       .
"001013" 2008    83.5
"001013" 2009    65.3
"001013" 2010    69.7
"001019" 1998       .
"001019" 1999       .
"001019" 2000       .
"001019" 2001       .
"001021" 1998   2.611
"001021" 1999   1.205
"001021" 2000    .477
"001021" 2001    .457
"001021" 2002      .5
"001021" 2003    .554
"001021" 2004    .397
"001021" 2005    .436
"001021" 2006    .697
"001021" 2007   1.022
"001021" 2008   1.968
"001034" 1998  36.034
"001034" 1999  40.168
"001034" 2000  43.276
"001034" 2001   86.65
"001034" 2002  67.088
"001034" 2003  63.232
"001034" 2004  81.466
"001034" 2005  26.936
"001034" 2006   44.43
"001034" 2007 140.255
"001036" 1998      17
"001036" 1999      29
"001036" 2000       .
end