# Probability of arriving at office before $9$ am

I'm having trouble answering this question: A person leaves for work between $8:00$ A.M. and $8:30$ A.M. and takes between $40$ and $50$ minutes to get to his office. Let $X$ denote the time of departure and let $Y$ denote the time of travel. If we assume that these random variables are independent and uniformly distributed, find the probability that he arrives at the office before $9:00$ A.M.

Any help would be appreciated.

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