Addition Formula

I am a new excel user. I a trying to write a certain formula but am having trouble. I searched this site and looked in the "Excel For Dummies" book that I have, but can't seem to find the help I need.

I want to write the formula to add a column of numbers, say H-10 through H-15. Each cell will have a number in it, but I want only to add the cells if the cell precedding it in the G-10 through G-15 Collumn is blank. For example if cells G-12 and G-14 have an "X" in them, then I do not want Cells H-12 and H-14 to be added. I only want the formula to add cells H-10,H-11,H-13, and H-15. I used just 6 cells for example, the column of cells to be added will be a lot longer.

Thanks