The departure board

A departure board is a table of clock times, and the arithmetic it asks for is elapsed time: a flight that leaves at 10:40 and takes 95 minutes lands at 12:15. Ninety-five minutes is one hour and thirty-five; 40 minutes and 35 minutes make 75, which is one more hour and 15 minutes.

The trap is that a clock carries at 60, not at 100. Children who add the numbers as if they were ordinary write 10:40 + 1:35 = 11:75, a time no clock shows. On MeasurePals they read a small board of invented flights and pick the landing time from four, one of which is exactly that trap.

Every time on the board is local. A flight that leaves at 9:00, flies 5 hours and crosses into a time zone three hours behind lands at 11:00 by the clock on the arrivals wall, though everyone aboard has lived through five hours.