Where is the mistake in my probability calculation for a 20cm string cut?

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The user is seeking clarification on a probability calculation regarding the lengths of string pieces cut from a 20cm string. They calculated the probability of the shorter piece being between 8 and 10 cm as P(8 ≤ X < 10) = 2 . (1/20) = 1/10, but their teacher indicated the correct answer is 2/10. The misunderstanding stems from the fact that the shorter piece does not necessarily include a specific mark, which affects the probability calculation. The user acknowledges this clarification. The discussion highlights the importance of understanding the conditions affecting probability outcomes.
songoku
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Homework Statement
Please see below
Relevant Equations
Continuous Uniform Distribution
1663238891159.png


I want to ask about part (c). This is what I did:

the length of the shorter piece should be 8 ≤ X < 10 so P (8 ≤ X < 10) = 2 . (1/20) = 1/10

But my teacher said the correct answer is 2/10. Where is my mistake?

Thanks
 
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The shorter piece of string does not necessarilly have the mark on it.
 
I understand. Thank you very much hutchphd
 
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