Depends on your question...
If your question is r + 0.12 = 0.20 then do this:
Subtract 0.12 from both sides
r = 0.08
The answer is B.
If your question is r - 0.12 = 0.20 then do this:
Add 0.12 on both sides
r = 0.32
The answer is A
So it depends on your question :P