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Bunuel
If a point is arbitrarily selected on a line segment, thus breaking it into two smaller segments, what is the probability that the larger segment is at least twice as long as the smaller one?

A. \(\frac{1}{4}\)
B. \(\frac{1}{3}\)
C. \(\frac{1}{2}\)
D. \(\frac{2}{3}\)
E. \(\frac{3}{4}\)

Did it in rather quick and crude method, on a scale of 100 (10 unit intervals):
10:90 - Yes (10X2 = 20)
20:80 - Yes
30:70 - Yes
40:60 - No
50:50 - No
60:40 - No
70:30 - Yes
80:20 - Yes
90:10 - Yes

So probability = 6/9 = 2/3
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hi brunel..

after going through the solution i got stuck at the last statement : The probability that either x≥2/3x or x≤1/3x is 2/3

how we can conclude probability is 2/3 and not 1/3.Both the options are given in the answer choices
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hi brunel..

after going through the solution i got stuck at the last statement : The probability that either x≥2/3x or x≤1/3x is 2/3

how we can conclude probability is 2/3 and not 1/3.Both the options are given in the answer choices


Look at the problem in this way.
Consider a line segment AB (any length)

Put two points on the line segment (say C and D), such that 2*AC=CB and 2*DB=AD
(i.e. the two points divide the line segment into 1:2 ratio)


Now, consider the question that you have to arbitrarily select a point on the segment.
Note carefully here that if you chose a point between A and C, or D and B, then it will satisfy the condition, which is that one line segment is greater than the twice of other.

Next, notice that C and D divide the line segment in a ration of 1/3 (i.e., AC=CD=DB=1/3*AB)

We also know that any point between C and D will not satisfy the condition

Therefore Probability of condition not true will be when the point is chosen between C and D, which is 1/3

Hence, probability of condition being true is 2/3 and answer is D

Hope this helps.
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can we also solve it this way

let the shorter side be x
then we assume, the longer side will be twice the shorter side i.e. 2x
so the probability of the longer side being twice as long as the shorter side will be 2x/2x+x
which is 2x/3x, which is 2/3 (after cancelling out the x's)
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I have edited the question and the solution by adding more details to enhance its clarity. I hope it is now easier to understand.
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I don’t quite agree with the solution. When a point is arbitrarily selected so why does the location matter?
Are there any identifiers for the larger and smaller segments other than the lengths?
questions asks about P(larger segment > 2 smaller one)
Now, if you choose the point in different segments, but the lengths are 3 and 7, it should be counted as one, not two, as mentioned in the solution.

Please clarify on this one
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D3N0
I don’t quite agree with the solution. When a point is arbitrarily selected so why does the location matter?
Are there any identifiers for the larger and smaller segments other than the lengths?
questions asks about P(larger segment > 2 smaller one)
Now, if you choose the point in different segments, but the lengths are 3 and 7, it should be counted as one, not two, as mentioned in the solution.

Please clarify on this one

Nothing is counted as two. Each point splits the segment into two lengths. We check if the longer one is at least twice the shorter one. That’s it. This happens when the point is in the first 1/3 or last 1/3, so the probability is 2/3.
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