The answer depends on whether you will be reusing the same two cells each time or will type a number into one of these cells then move on to the next row for the next time. Both cells need to be formulas. Once you overwrite a formula with an actual number, it is gone.
If you will be using them one time only,
B2 =IF(ISERROR(FORMULATEXT(C2)),OFFSET(B2,0,1)×5,"")
C2 =IF(ISERROR(FORMULATEXT(B2)),OFFSET(C2,0,−1)÷5,"")
Fill down with both to complete the columns.
Type a number into either cell (overwriting the formula) and the other will do its thing.
The use of OFFSET tricks it to not see the circular reference. In reality, it isn't really circular because the part of the IF statement that references the other cell for multiplication/division only gets executed after the other cell's formula has been overwritten with a value.
If you want to reuse the same two cells then you will need to put your input into a third cell so you do not overwrite the formulas. An entry might be something like 5L or 5R to specify which cell gets the "5" and which one gets either 25 or 1/5. One possible solution would be this:
A2 is where you enter the number followed by either an L or R
B2 =IF(RIGHT($A2,1)="L",LEFT($A2,LEN(A$2)−1)×1,LEFT($A2,LEN(A$2)−1)×5)
C2 =IF(RIGHT($A2,1)="R",LEFT($A2,LEN($A2)−1)×1,LEFT($A2,LEN($A2)−1)÷5)
