Question 1077635
The quartiles can be found from the range of speeds:  115-55 = 60 and 60/4 = 15, so the quartiles are:

 55-70   1st quartile
 70-85   2nd quartile
 85-100 3rd quartile
 100-115 4th quartile   <<< Part (c) indicates driving in this quartile ("above the 3rd quartile") is most dangerous

Speed limit should be set at 100.  

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Note that the median is used to find the middle of the data (and then quartiles are determined based on an ordered set of the data, 1/4 on bottom, 2nd 1/4 next group, 3rd 1/4/ next group, 4th 1/4 highest group), in this case I used the mean, but the problem states the speeds driven are uniformly distributed so this works.