PreCalculus
posted by Mary .
The table below shows a frequency distribution of the number of pickup trucks sold at 85 truck dealerships in Maine over an 18month period.
Number of trucks sold/Number of dealerships:
7090/2
90110/11
110130/39
130150/17
150170/9
170190/7
Find the interval about the sample mean such that the probability is 0.90 that the true mean lies within the interval. (When P=90%, t=1.65)
I know that N=85, but I do not know how to find the standard deviation or mean in order to work the rest of the problem. Any help would be great.

Frankly, I would assign numbers to do the mean, standard deviation.
Change your frequency to actual data
80,80, 100,100,100,100,100,100, (eleven 100's), 120 (39 of these). Your calculator can be your best friend doing this.
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