Your question is Grandmothers Train Paradox. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You have two grandmothers living in Manhattan—one in the north and one in the south. There are six trains that run between 4 PM and 5 PM: three go north and three go south, alternating in their schedule. Each day, you arrive at the train station randomly between 4 PM and 5 PM. At the end of the year, you realize that you've visited your grandmother in the north 70% of the time, while you would expect it to be 50%. Why is this the case?