Weihnachtsmann rot

Challenge from 16. December

Rendezvous at New Moon

Author: Khai Van Tran

Challenge:

As every new moon, the elves Yann and Max are appointed for a night walk and a subsequent puzzling night. Both elves live on the (almost infinitely) long and totally straight Christmas Street. Since they live two miles apart, they usually leave an hour before their appointed meeting time and walk one mile towards each other. After one hour, they meet in the middle of their houses (you wouldn't be faster on these little elf legs).

Today, they also leave their houses at exactly the same time. But, unfortunately, there has been a tropical storm (the climate change does not spare anyone), that has taken all the road signs and destroyed the information and telecommunications system.
Yann and Max can distinguish the two directions in which the road is heading; however, they do not know in which direction the other elf lives—the Christmas stress must have made them oblivious. Since it is a pitch-black night, they cannot use the sun's position for orientation. Furthermore, they cannot count on the North Star, as they already are quite near to the North Pole. Light and smoke signals are not a possibility either, since Yann and Max are not to disturb they neighbours on Christmas Street—in short, they cannot communicate with each other...

  1. Each elf tosses a fair coin to determine in which direction to go for one mile. This procedure is repeated until the elves meet.

  2. First, each elf tosses a fair coin to determine in which direction to go for one mile. If they do not meet, then each elf turns and goes on for one mile. This procedure is repeated until the elves meet.

  3. First, each elf tosses a fair coin to determine in which direction to go for one mile. If they do not meet, then each elf turns and goes on for two miles. This procedure is repeated until the elves meet.

Since Yann and Max are bosom buddies, they can be absolutely sure to pick the same strategy. The question remaining is, how good each strategy is.
Leta, b, and c be the average time (measured in hours) until the two elves meet if they choose strategy A, B, and C, respectively.

Which of the following statements is true?

Artwork: Friederike Hofmann

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Possible answers:

  1. a < b < c

  2. a < c < b

  3. a < b = c

  4. b < a < c

  5. b < c < a

  6. b < a = c

  7. c < a < b

  8. c < b < a

  9. c < a = b

  10. a = b = c

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