TOPIC: EAC when to use which formula

I was doing a practice exam and knew the BAC, AC and EV. The question asked to calculate the EAC. I used the formula EAC = AC + BAC - EV. and found my answer. However it was marked wrong and instead the solution used EAC = BAC/CPI . Is there a way to determine when to use one formula vs the other?

here are the notes from my study log, hope t helps

If the CPI is expected to be the same for the remainder of the project, EAC can be calculated using: EAC = BAC/CPI
which basically means you expect the future of the project to be the same as the previous period.

If future work will be accomplished at the planned rate, use: EAC = AC + BAC – EV
which means whatever happened in the past period will not affect the plan of the remaining work.

If the initial plan is no longer valid, use: EAC = AC + Bottom-up ETC
which means the initial planning turned out invalid and the remaining work will be re-estimated.

If both the CPI and SPI influence the remaining work, use: EAC = AC + [(BAC – EV)/(CPI x SPI)]