The teachers' union in california wants to know the average salary for high school teachers throughout the country. what is the teachers'union presumably planning to calculate?

Mean or most likely, sample mean

Step-by-step explanation:

Why ?

The mean is the central tendency of a data set, and it calculated by taking the sum of the of observations, and divided by the number of observations.

e.g. average age of students in a particular class, average height of students in a class etc.

but Sample Mean, which is the most applicable type of mean calculation occurs when the number of possible observation is too large e.g. height of all men in the world, or age of every student in the country or even number of everyone in the US. And in this case, the number of high school teachers throughout the country. Why? because we would have to get the salary of every single high school teacher and more often than not, it's impossible.

so enter "The Sample Mean" simply put we take a portion of the population (high school teachers) and we divide it by the no of observations we have. and if our sample is random enough, it will give us a good unbiased estimate of what we are looking for, in this case average salary of high school teachers.

expression #3 is not equivalent. the others are.