The first one is to calculate returns, the second one for duration. May be that the other one could work, but especially for A.M. I would remember both and use the right one. May give more points.
Yes you’re correct but I was still able to get the correct answer using the incorrect formula. i.e. Reading #23 question 2 I used the first fomula when trying to calculate duration and I still got 14.4 for duration.