PrepTest 151 · Section 1 · Passage 4

The Mystery of the Quiet Hallway

Everyone at school accepts Ms. Rivera's Bump Theory: most dropped trays happen when kids crowd together at hallway merges and bump. The more merging, the more drops. Most drops do happen at busy merges. But here is a puzzle: some busy merges, like the one by the library, have almost no dropped trays. How can lots of merging happen with so few drops?

Priya's group has an answer. At the busy, drop-heavy merge by the gym, two lines walk toward each other, so the spot where they meet stays put. At the library merge, both lines walk the same way, and the back line is just faster, so its front kid squeezes in. But that meeting spot slides along with the crowd. So, like a kid stepping off a moving merry-go-round, the faster kid gets jerked and cuts in at a sharp angle. A sharp cut means only a small patch of tray touches tray, so there is little rubbing and few spills. When lines walk toward each other, the kids slide past at a low angle, so their trays touch along a long edge. Like two strips of velcro, they grab and tip.

Priya adds a warning. Hallways everyone thought were safe because they have very little merging might still see plenty of drops, depending on what kind of merging happens there.

The Case of the Calm Canal

Judge Okafor heard a boating dispute. Captain Gray testified, 'Everyone knows boats scrape when two meet in a narrow canal. The busiest canals have the most scrapes.' Mr. Whitfield raised a finger. 'Then explain the busy East Canal, Your Honor, with hardly a scratch.' He called an engineer. 'In scrape-heavy canals,' she said, 'boats come at each other, so they ease past at a low angle and rub along their whole sides. In East Canal, boats travel the same way; a fast one overtakes at a sharp angle and touches only a corner.' Mr. Whitfield beamed. 'So the kind of meeting, not just the amount, decides the scrapes. And a quiet canal is not automatically safe.' The judge wrote: puzzle, mechanism, warning.