Why don't we measure spread about the mean by simply averaging x − x , the deviations of individual data values from their mean?

Because this spread will add up to zero

Step-by-step explanation:

Mean defines the center of the data. Some values are below this center and some values are above this center.

For the values which are below the center (i.e. less than the mean), when mean is subtracted from these values, this results in negative numbers.

For the values which are above the center(i.e. greater than the mean), when mean is subtracted from these values, this results in positive numbers.

When these negative and positive numbers are summed together, as it is, they cancel out each other, leaving an answer equal to zero, which would be meaningless. As a spread of 0 would mean all data values are the same, when infact they aren't.

To avoid this, the difference of mean from the data value is squared, so that we can get all positive values, and then these values are added up together to calculate the spread of the data.

Conclusion:

The average of deviations of individual data from the mean will result in answer equal to zero, as a result they are squared first before finding the average.

answer: 590

step-by-step explanation:

a) one student is chosen from classroom a, then a student is chosen from classroom b.  

step-by-step explanation:

independent events are ones in which the probability of one event does not affect the probability of the second event.

one student is chosen from classroom a; this does not affect the chances of choosing a student from classroom b.   this makes these independent events.

when one card is chosen from a standard deck and then set aside before a second card is chosen, the probability of the second card being chosen changes.   this makes them dependent events.

when one student is chosen from classroom a and then a second student is chosen from classroom a, the probability of choosing the second student changes.   this makes them dependent events.

when a letter is chosen from the alphabet and then a second letter is chosen, the probability of choosing the second letter changes.   this makes them dependent events.