TOPIC: Why is this the correct answer?

Hi,

I don't understand how this answer was calculated for the question from Oliver Lehmann's free 75 question test.
Can anyone please enlighten? The correct answer is B ie. approx. 5.2 days.

27. A project manager made 3-point estimates on a critical path and found the following results: (Check attached table below)

Assuming ±3 sigma precision level for each estimate, what is the standard deviation of the allover path?

(A) App. 4.2 days
( App. 5.2 days
(C) App. 6.2 days
(D) You can not derive the path standard deviation from the information given.

You can use following link for calculation:
www.firebrandtraining.co.uk/learn/pmp/co...e-activity-durations
You have to calculate standard deviation for each activity.
(24-12)/6=2,
(14-/6=1,
(27-15)/6=2,
28-10)/6=3,
(35-17)/6=3
Variances of each activities are squares of deviations of activities.
Stand.deviation of path is square root of sum of variances:
SQRT(4+1+4+9+9)=SQRT(27)=5,19

For +_ 3 Sigme, the standard deviation = 3 * Standard Deviation of 1 Sigma prceision level.

So in this case, the standard deviation should be = 3 * 5.19 = 15.57

Hope, I am right.
If not, PLEASE explain me.

Waw...Super..I got it...thanks a lot...dint understand logit behind t ...but doesnt matter

please note that putting ±3 sigma precision level in the question is meaningless and useless it is just to scatter you away

I didn't understand y it is 5.2 days also what's the logic behind this.
SD=P-O/6
Could you please explain?

So how to solve this problem?

@Ivan Fućak, thank you very much for that answer. 4 years later and your answer is sitll helping out folks. Much appreciated!