I have a Variable in Cell A1, in this case it's 20

The Variable is to adjust a percent increase or decrease of a fixed amount

If my fixed value in cell B1 is 10 and I want to decrease it by 20% in cell C1, currently I am getting 2

So I have to use another formula to subtract 2 from 10, and I get 8

What is the formula to make the value in C1 8 instead of 2

I also need the reverse formula to increase it in another cell range by that same variable in A1

So 10 becomes 12 if the increase is 20%

That's the first phase, next I'll show a specific round up or round down requirement that is rather complex to.

Thanks