PMBOK, pg. 267 is the key resource.

Kyle is right...

If I see something about "...at the budgeted rate", use EAC = AC + BAC – EV. If I see something about "...the costs index will remain constant", then use BAC/CPI. If I see CPI and SPI (given in the question) or something about "...cost and schedule must be maintained", use the CPI*SPI formula.

Also, I think candidates want to study the formulas and just expect the information to be easily plugged from the question. However, it's important to study the formulas, but even better to understand the relationships between EAC, ETC, VAC, CPI and SPI.

All these formulas are alebraic, ETC = EAC-AC and EAC = AC + ETC (new), these formulas are shown in both ETC and EAC, but are just algebraic re-arrangements. Prepcast does a good job of testing the correct use of the formulas, because as I recall a couple of the questions needed two or three steps to completed (re-arranging related formulas to find missing numbers to plug into a final solution).