Formula Problem Breakdown

Here is a revision to my original problem.

The following are plug in numbers and a formula answer is derived from that, as follows:

Cell 1(c1) Cell 2(c2) Answer(A)

The formula base takes the information put into cell 1 and performs basic math using the cell reference c1 as a component.
The same situation applies to cell 2.
Example:

SUM((c1/2)+3.875)
SUM((c2+1.625)*5

These functions generate an answer(A) by the sum of a column.
Example:

Cell (A) =SUM B5:B17

I now want to plug into the original c1 & c2 some new variables to calculate different answers(A). This currently does not work even if the new plug in listing is within the same sheet.
Example:

c1 c2 Answer(A)

SUM((c1/2)+3.875)
SUM((c2+1.625)*5

New plug in list to use in original formula
c1= c2= New Answer (NA)
10 10 ?
15 15 ?
20 20 ?

Is there a way to insert the new plug in listing into the original c1 & c2 cell reference to generate a new answer?

Thanks to everyone for the help.