Way off subject

["Chuck Nicodemus" <armorseal@spiretech.com>]

Now that the car is running great and the asphalt season is drawing to a
close. I am working on some spread sheets for work. My dilemma is this.
If I have an amount in one cell, and want to add more to that amount in
that same cell. What is the formula I am working in Excel and it is
compatible with 123 and others. 
A closer example is 
10 x 500 = 5000
but I want to be counting up in the 10  area like add 15 to it.
 
Anyone ? 
Thanks and you can address it to me off list.
Chuck perplexed.