Edward,
May I suggest our PMP Exam Formula Study Guide at
www.pmformulas.com
?
I'm not trying "to make a sale" here, but if you are having trouble understanding the EV formulas, then the formula study guide is a great tool where you have all the formulas in one place.
1) Here is what the formula guide says about these formulas
ETC = EAC - AC: Inversion of the same formula from the EAC calculations. Note: This ETC formula is listed in only one (the “most famous”) PMP prep workbook. No others list it. We recommend using it, if no keywords are given.
ETC = BAC - EV : Assumption: use formula if current variances are thought to be atypical in the future.
ETC = (BAC - EV) / CPI :Assumption: use formula if current variances are thought to be typical in the future.
2) Here is a sample question from our formula study guide:
Question 10: PV = 100, EV = 105, BAC = 400, AC = 102, CV = 3, ETC = 300, EAC = 407. The ETC is given as 300. However, how much would the Estimate to Complete be if we assume that current variances are thought to be atypical?
and the answer is of course BAC-EV