As Steven Spielberg would have it, today's the day Marty McFly visits the far-flung future of two thousand fifteen. I wrote down a time-travel themed Advanced Knowledge Problem, but I left it in the trunk of my flying car. Here's the one I came up with in the meantime.
Think you've worked it out? Keep reading for my solution.
Did you find the answer this way, or another way? Make sure to leave a comment if you found this problem interesting!