Add monthly totals of months forward

[mikeinrbs]

Under the col of hours i need to add the hours from that month forward. the
months and monthly totals are in the same workbook, but different sheets. for
example, an enrty of sep09 needs to add all totals through apr2010, however,
an entry of nov2009 under installed needs nov 2009 forward, not including the
previous months. suggestions????
Sep-2009 Oct-2009 Nov-2009 Dec-2009 Jan-2010 Mar-2010 Apr-2010
11,500,000 11,500,000 11,500,000 11,500,000 11,500,000 11,500,000 145,000,000


Hours Installed
1-Sep-09
1-Oct-09
1-Nov-09
1-Dec-09
1-Jan-10
1-Feb-10
1-Mar-10
1-Apr-10
1-May-10
1-Jun-10
1-Jul-10
1-Aug-10