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2 years, 7 months ago

What is the probability?

The names of the seven days of the week are placed in a hat. Three names are drawn out; these will be the days of the Probability I lectures. What is the probability that no lecture is scheduled at the weekend?
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opher | 2 years, 7 months ago
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You need to multiply the likelihood that only non-weekend days will be drawn each of the three times. However, the number of day names left in the hat decreases with each drawing, reducing the denominator in each term. At the same time, the number of non-weekend days is also reduced with each successful (i.e. non-weekend day) drawing, reducing the numerator by one each time as well. The calculations is thus as follows.

5/7 * 4/6 * 3/5 = (5 * 4 * 3)/(7 * 6* 5) = 2/7 or about 28.57%.

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rajvaidhy | 2 years, 7 months ago
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Think of names being drawn one at a time and done 3 times
The 1st draw is a non-weekend is 5/7 -- as there 5 non-weekend days
The 2nd draw is a non-weekend is 4/7
The 3rd draw is a non-weekend is 3/7

So the required probability is (5/7)*(4/7)*(3/7) =

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albanian | 2 years, 7 months ago
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5/7*5/7*5/7= 0.364431487
There is a 36% chance of this happening (rounding for clarity). Or looking at it the other way, there is a 64% chance your weekend will be spoiled. Don't leave scheduling to chance!

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albanian | 2 years, 7 months ago Report

My answer is only correct if the days drawn are returned to the hat after drawing. This would allow more than one lecture per day. The other answers are correct if the days drawn are not returned to the hat and only one lecture is allowed per day.

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abbiewilson55 | 2 years, 7 months ago
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Here the sampling is without replacement, and we can take it to be either
ordered or unordered; the answers will be the same. For ordered samples, the
size of the sample space is 7P3 = 7 • 6 • 5 = 210. If A is the event ‘no lectures at
weekends’, then A occurs precisely when all three days drawn are weekdays; so
|A| = 5P3 = 5 • 4 • 3 = 60. Thus, P(A) = 60/210 = 2/7.
If we decided to use unordered samples instead, the answer would be 5C3/7C3,
which is once again 2/7.

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