Pixie, Kiki, Bill and Bets have gone for an adventurous trip. On the way they must cross a old rustic footbridge at night. Many planks are missing, and the bridge can hold only two people at a time (any more than two, and the bridge collapses).
They must use a flashlight to guide their steps otherwise they're sure to step through a missing space and fall to their death. There is only one flashlight.
Each of them can travel at different speeds. Angry Pixie can cross the bridge in one minute; Kiki in two minutes; Bill takes five minutes; and the slowest person, Bets, takes ten minutes. The bridge is going to collapse in exactly seventeen minutes. How can all four people cross the bridge?