If you can figure out these formulas in your head / on paper, then I say "more power to you!".
However... On the PMP Exam you have just 1.2 minutes to answer each question. So don't make this harder on yourself than necessary. During the exam you can use an on-screen calculator (windows based) and you don't have to calculate anything by hand.
In fact, I recommend that you use the on-screen calculator even if you have to calculate 2*5=? because the exam is a stressful time and you could easily make a mistake and then 2 times 5 is suddenly 12.