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Homework Statement
Jake leaves home at a random time between 7:30 and 7:55 a.m.
(assume the uniform distribution) and walks to his office. The walk takes 10
minutes. Let T be the amount of time spends in his office between 7:40 and
8:00 a.m.. Find the distribution function F_T of T and draw its graph. Does F_T
have a density?
Homework Equations
The Attempt at a Solution
This is what I've come up with so far:
Let X be the number of minutes past 7:30 that he leaves his house.
[tex]T = \begin{cases} 30 - (x + 10) &\text{if } 0\leq x \leq 20 \\ 0 &\text{if } 20<x\leq 25 \end{cases}[/tex]
[tex]F_T(t) = \mathbb{P}[T\leq t] = \begin{cases} \mathbb{P}[20-x\leq t] &\text{if } 0\leq t \leq 20 \\ 0 &\text{if } t<0 \\ 1 &\text{if } t>20\end{cases} [/tex]
[tex] \mathbb{P}[20-x \leq t] = 1 - \mathbb{P}[x<20 -t][/tex]
[tex]= 1 - \int_{0}^{20-t} \frac{1}{25} dx[/tex]
[tex] = \frac{5+t}{25}[/tex]
[tex]\therefore F_T(t) = \mathbb{P}[T\leq t] = \begin{cases} \frac{5+t}{25} &\text{if } 0\leq t \leq 20 \\ 0 &\text{if } t<0 \\ 1 &\text{if } t>20\end{cases}[/tex]
This would imply no density, but that seems plausible given that for [tex]T=0[/tex], [tex] 20<X\leq 25[/tex].