AMC 10 Step by Step
2022 AMC 10BProblem 23P21-25~6 minPrint

Ant Amelia starts on the number line at 00 and crawls in the following manner. For n=1,2,3,n=1,2,3, Amelia chooses a time duration tnt_n and an increment xnx_n independently and uniformly at random from the interval (0,1).(0,1). During the nn th step of the process, Amelia moves xnx_n units in the positive direction, using up tnt_n minutes. If the total elapsed time has exceeded 11 minute during the nn th step, she stops at the end of that step; otherwise, she continues with the next step, taking at most 33 steps in all. What is the probability that Amelia’s position when she stops will be greater than 11 ?

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Problem © Mathematical Association of America (MAA), American Mathematics Competitions. Reproduced for non-commercial educational use. Solution and commentary are original to this site.