Summing row values ONLY to the left of a row, not the right..

I am lending someone $100 on month 1, $200 on month 2 and $300 on month three and so on. I charge the guy 10% interest. I want to be able to show how much his monthly interest charge is. On month 1 it is 10% x 100, but month two it's $300 x 10% and month 3 it is $600..

I guess I am asking what formula or function will add together multiple columns of the same row, but to the left only. So if I am on E30, I want to calculate SUM=B30+C30+D30. But if I am on F30, I want the cell to know to automatically do =SUM=B30+C30+D30+E30..

Thanks!

Put this in E30:

=SUM($B30:D30)

Then if you copy this into F30 it becomes:

=SUM($B30:E30)

and so on.

Hope this helps.

Pete