Riddle: You, Debbie, David and Geoff are being chased by zombies. The zombies are 17 minutes behind when you reach canyon that has only 1 crossing: a bridge. Only two people can cross the bridge at a time, and you must all reach the other side before the zombies reach the bridge (or else they will go onto it and anyone on it will die as it collapses). You can cross the bridge in 1 minute, David can cross in 2 minutes, Debbie takes 5 minutes and Geoff takes a whole 10 minutes to cross. To make things worse it is dark and you have to cross using the one lantern you have between you. Each person crossing has to have the lantern or be next to the person with the lantern. However, any of you can safely wait on either side in the dark. How do you all get across before the zombies reach you?
Answer: You and David cross, taking 2 minutes. David waits while you cross back with the lantern, taking another minute (3 minutes total). Debbie and Geoff cross together, taking 10 minutes (13 minutes total). David, who has been waiting, takes the lantern and crosses back in 2 minutes (15 minutes). The two of you cross together taking 2 minutes (17 minutes total). Congrats, you made it!