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A commuter finds that she waits an average of 18.5 seconds at each of four toplights, with a standard deviation of 6.5 seconds. Find the mean and the standard deviation of the total amount of time she waits at all four lights.​ What, if​ anything, did you​ assume?

A. The mean time she waits at all four lights is  74 seconds.

b.  The standard deviation of the time she waits at all four lights is  13

It is necessary to assume the wait times at the lights are independent to calculate the standard deviation.

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