I have the following case

A B C
1 Date Value Mark
2 23.10.2007 100 X
3 23.10.2007 150
4 23.10.2007 200
5 23.10.2007 300 X
6 24.10.2007 123
7 24.10.2007 344
etc...

Now I would like to have a formula which would sum the values on a
certain date, that are marked with an X. For example, the result on
23.10.2007 would sum only 100+300=400