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# Need help (statistics)?

A bus scheduler studied the time taken to travel a particular route during off-peak times over a period of several months. He found that the time taken for the outbound journey had a mean of 28.5 minutes and a standard deviation of 3.2 minutes, while the time taken for the inbound journey had a mean of 30.1 minutes and a standard deviation of 2.6 minutes. The scheduler is interested (among other matters) in the difference between the times taken to complete the outbound and inbound legs of a round trip. Define D such that it is “outbound trip” ― “inbound trip”. Determine the standard deviation of the variable D.
by Bail Out
Member since:
February 01, 2011
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20,720 (Level 6)

## Best Answer - Chosen by Asker

If X and Y are two independent random variables,
V(X+Y) = V(X-Y) = V(X)+V(Y)

And so, if the variables defined (“outbound trip” and “inbound trip”) are independent
V(D) = V(outbound) +V(inbound trip) =3.2^2 + 2.6^2 = 17.
SD(D) = sqrt(17) = 4.123
• 1 person rated this as good
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Asker's Comment:
Thank you so much!
Can you help me with this other question too:
http://uk.answers.yahoo.com/question/ind…

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