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Problem-Solving and Data Analysis / Probability and conditional probability Difficulty: Easy

	Live east of the river	Live west of the river	Total
Less than $40$ $40$ years old	$17$ $17$	$11$ $11$	$28$ $28$
At least $40$ $40$ years old	$18$ $18$	$89$ $89$	$107$
Total	$35$	$100$	$135$

The table summarizes members of a local organization by age and whether they live east or west of the river. If a member of the organization is selected at random, what is the probability that the selected member is at least $40$ years old?

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Explanation

Choice D is correct. If a member of the organization is selected at random, the probability that the selected member is at least $40$ years old is equal to the number of members who are at least $40$ years old divided by the total number of members. According to the table, there are a total of $135$ members of the organization, and $107$ of these members are at least $40$ years old. Therefore, the probability that the selected member is at least $40$ years old is $StartFraction 107 Over 135 EndFraction$ .

Choice A is incorrect. This is the probability that the selected member is less than $40$ years old.

Choice B is incorrect. This is the probability that the selected member lives east of the river.

Choice C is incorrect. This is the probability that the selected member lives west of the river.