Probability of Visiting Grandparents
The schedules of the trains in the two directions are shifted relative to each other:
If, e.g., on the red line, the trains arrive at $3, 6, 9,$ etc. minutes to an hour, while on the blue line that's $1, 4,7,10$ minutes, etc. then the chances of the boy catching the red direction are $2$ to $1.$
The pdf (probability distribution function) of his arrival is such that the ratio of the areas under the pdf for time intervals corresponding to the two direction is ~$2.$
This is an old problem that I heard of yet in high school. Can't pinpoint the source. Solution 2 is by Amit Itagi; Solution 3 is by Elia Noris.