Question on test-standard deviation

[rudyx61]

Question on test---standard deviation

Was hoping someone could help me out with a question. It was on a online test and the answer i got i wasnt sure if it was the right answer.

The question was as follows:

The no-show rate for passengers with reservations on a flight run by wizair is 16%. The next flight has 42 reservations.

Find the standard deviation of the number of no-shows for this flight.

Answers:

A-5.940
B-5645
C-2.376
D-2.592
E-35.28
F-6.72
G-4.218
H-3.124

what i did was calculate the mean number of no-shows for the flight

(16%)*42=6.72

and to calculate the standard deviation for number no-shows on that flight i took the square root of the mean number of no-shows for the flight

sqrt(6.72)=2.592

so what I've done is it correct?

 

[uart]

No rudyx16, that would be wrong because the stdev is the sqrt of the variance, not the sqrt of the mean as you have done.

Interestingly the mean and the variance are numerically quite similar in this problem, so numerically your answer is close but still wrong.

This is a case of a Binomial Distribution, http://en.wikipedia.org/wiki/Binomial_distribution , which has a mean = n p and variance = n p q.

Here n is the number of passengers (42), p is the probability of "no show" (0.16) and q is the complementary probability (0.84).

So the correct answer is sqrt(42 * 0.16 * 0.84) which corresponds to multiple choice "c".

 

[rudyx61]

hmm, i nearly got away with the wrong answers. thanks for the help