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

[songoku]

Homework Statement

Please see below

Relevant Equations

Continuous Uniform Distribution

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

 

[hutchphd]

The shorter piece of string does not necessarilly have the mark on it.

 

[songoku]

I understand. Thank you very much hutchphd