Is this possible?

I'm working on a food waste program through excel and the issue im having right now is when I insert a simple formula like =(C2*D2) it will correctly answer the forumla but im trying to figure if i can erase the # in D2 after and have it go back to zero?

Sorry if im not explaining this well..my first time actually using excel so im struggling.
Below is a picture of what im doing

\1

so here's my problem in picture form..so i entered the formula and the number and it does the math right as you can see
http://i42.tinypic.com/15hxs7o.png
but..when i remove the five the E2 never changes when i want to go to to zero
http://i44.tinypic.com/34tbqeb.png

First make your title accurately explain what ur trying to accomplish, next up if you can go to edit, advanced, and upload your workbook. But from what I can see use =SUM(C2*D2) and see if that works

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