Me again,
I found a similar problem and explanation which helped me figure out this one.
You must first calculate the standard deviations of each sequence activity, A through E, using (P-O)/6. Square each standard deviation to yield the variance (Variance = SD^2). Add all the sequence activity variances together then take the square root of the sum. This will yield 5.19... which rounds to 5.2. You could then express the schedule as 83.1 +/- 5.2 days.
I just passed the PMP today but couldn't let this question go.
Thanks