# Thread: Determining SD from percentage

1. ## Determining SD from percentage

Hi all,

This is a question from my mid-session test, I can understand most of the others I got wrong, but I have no idea how to answer this.

Use the following to answer questions 14 & 15
JETSTAR's daily flight from Sydney to Broken Hill is on time seventy per cent of the time. Over the next 10 days:

Q15. The standard deviation of the relevant probability distribution (number of flights on time) is (closest to):

a) 1.45,
b) 2.10,
c) 3.00,
d) 7.00,
e) Impossible to calculate from the information given.

I'll avoid giving away my thinking to stop eliminating potential answers (because I got it wrong). Thanks folks.

P.s. This is all the information for the question.

2. Originally Posted by Peleus
Hi all,

This is a question from my mid-session test, I can understand most of the others I got wrong, but I have no idea how to answer this.

Use the following to answer questions 14 & 15
JETSTAR's daily flight from Sydney to Broken Hill is on time seventy per cent of the time. Over the next 10 days:

Q15. The standard deviation of the relevant probability distribution (number of flights on time) is (closest to):

a) 1.45,
b) 2.10,
c) 3.00,
d) 7.00,
e) Impossible to calculate from the information given.

I'll avoid giving away my thinking to stop eliminating potential answers (because I got it wrong). Thanks folks.

P.s. This is all the information for the question.
Probability that a flight is on time is $0.7$, assume that the timeliness of one flight is independent of that of any other. Then the number on time has a binomial distribution $B(0.7,N)$ where $N$ is the number of flights involved. You should know what the SD of this is.

CB

3. Cool thanks - got it.

My problem was more reading the question. Just now it has clicked that it was a daily flight, I thought it was the total flights over 10 days, with the amount of flights being unknown.

As a result, I said (e), because I didn't have enough information to answer the question.

In fact N = 10 and I should be able to calculate it from there.

Thanks again.