This sounds easy but i do nt know...

I have a cell in which it holds a value of qauntity. as i
am counting hte qaunity from page to page from my data (
paper source) i want it to add each time i out the new
value in the same cell.
is htere a funtion or a macro is needed??

ie:

cell A2 has value 10..
i found another 15 of the same item and want to add it to
the 10 in the cell.
the value now holds 25.
I want it to where the 10 is in ther and i just put in 15
and then hit add or something to give me the new value.
like you would on a calculator.
isit possible? i ahve large numbers and many pages to go
thru.

many thanks!