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Question 190:

1

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I've coded your date into the contingency table below.

 Electric Usage kWh  Single Family  Multi- Family  Total
  < 5,000  15  25  40
 5,000 7,000  35  15  50
 7,000 11,000  65  15  80
 11,000 15,000  25  5  30
 Total  140  60  200

For question a) there were 80 households that had electric usage between 7000-1100kWh out of a total 200 households, giving the probability of 80/200 = .40.

For part b) the probability of selecting someone who uses less that 5000kwh is 40/200 = .20.

For part c) the use of the word OR indicates we need to add probabilities (the addition rule), the next question is if the events can happen at the same time (mutual exclusivity). Since you can be both a single-family home and use between 5,000 and 7,000 kWh, there is not mutual exclusivity. To take that into account, we calculate the individual probabilities, then subtract their joint probability. The probability of a single family dweller is 140/200 =.70. The probability of kWh usage between 5,000 and 7,000 is 50/200 = .25.  The probability of their joint occurrence is the cell in the table for both Single Family and 5000-7000kWh = 35/200 = .175.  So (.70 + .25)  - .175 = .775.

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