Use previous value in formula

I have searched everywhere and cannot find a solution. I'm not even sure it is possible, but this is what I'm trying to do. In cell A1 I will enter data. I want to subtract that value from cell B1 and put the result into C1. That by no means is my issue. Here is what I want. I want to use the value that I typed in A1 as my new value of B1, and enter a new value into A1. The result needs to stay in the same cell C1. Maybe this will help? This is what I currently have: A1 Data, B1 Data, C1 =mod(b1,100)-mod(a1,100). I need the value from A1 to be placed into B1 and then enter a new value into A1, with the result remaining in C1. Thank you to anyone who can tell me how, or if it is possible.

Ray

I have this question posted on MrExcel.com also. in the excel questions section.

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