Wednesday, February 16, 2011

Quick Bridge-Crossing

Four people come to an old bridge in the middle of the night. The bridge is rickety and can only support 2 people at a time. The people have one flashlight, which needs to be held by any group crossing the bridge because of how dark it is.

Each person can cross the bridge at a different rate: one person takes 1 minute, one person takes 2 minutes, one takes 5 minutes, and the one person takes 10 minutes. If two people are crossing the bridge together, it will take both of them the time that it takes the slower person to cross.

Unfortunately, there are only 17 minutes worth of batteries left in the flashlight. How can the four travellers cross the bridge before time runs out?


Anonymous said...

Lets say A takes 1 min,
B takes 2 min,
C takes 5 min,
D takes 10 min.
First AB will cross the bridge and A will come back (2+ 1 min)
second CD will cross the bridge and B will come back (10+2 min)
last AB will cross the bridge (2 min)
Total 15 min

Anonymous said...

sorry total not 15 min its 17 min(2+1+10+2+2)