AHA! Puzzle This
By Michael H. Pryor
Welcome to the first installment of AHA!, a page of MAKE’s favorite puzzles,
selected by Michael Pryor. When you’re ready to check your answers, visit
makezine.com/05/aha.
The Rope Bridge (Easy)
Four people need to cross a rickety rope bridge
to get back to their camp at night. Unfortunately,
their only flashlight has just 17 minutes of juice
left. The bridge is too dangerous to cross without
a flashlight, and it’s only strong enough to
support two people at any given time.
Each of the campers walks at a different
speed. One can cross the bridge in 1 minute,
another in 2 minutes, the third in 5 minutes,
and the slowpoke takes 10 minutes to
cross. How do the campers make it across
in 17 minutes?
Yarrr Maties (Sinister)
Five pirates discover a chest full of 100 gold coins. The pirates are ranked by their years of service, Pirate 5
having five years of service, Pirate 4 four years, and so on down to Pirate 1 with only one year of deck
scrubbing under his belt. To divide up the loot, they agree on the following:
The most senior pirate will propose a distribution of the booty. All pirates will then vote, including the
most senior pirate, and if at least 50% of the pirates on board accept the proposal, the gold is divided as
proposed. If not, the most senior pirate is forced to walk the plank and sink to Davy Jones’ locker. Then the
process starts over with the next most senior pirate until a plan is approved.
These pirates are not your ordinary
swashbucklers. Besides their
democratic leanings, they are also
perfectly rational and know exactly
how the others will vote in every
situation. Emotions play no
part in their decisions. Their
preference is first to remain
alive, next to get as much gold
as possible, and finally, if given
a choice between otherwise
equal outcomes, to have fewer
pirates on the boat.
The most senior pirate thinks
for a moment and then proposes
a plan that maximizes his gold,
and which he knows the others will
accept. How does he divide up the
coins? And what plan would the most
senior pirate propose if the boat had 15
pirates instead of just five?
Michael Pryor is the co-founder and president of Fog Creek Software. He runs a technical interview site at
techinterview.org.
Illustrations by Roy Doty
