I have spread sheet with a 3 columns
1. column contains the projectnumber(1,2,3 etc)
2. column contains the week
3. column contains the cost made per week and per project(manually entrie)
4 column should sum all the costs which are made in the different weeks.

I dont want to have totals per project. If the costs made for several
projects in one week, eg week 30 will appear 3 times (entry per
projectnumber) , than I want to have a total of all costs made per week. (sum
of the 3 entries )