Riddle #793


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?
The two keys here are: You want the two slowest people to cross together to consolidate their slow crossing times. You want to make sure the faster people are set up in order to bring the flashlight back quickly after the slow people cross. So the order is: 1-minute and 2-minute cross (2 minute elapsed) 1-minute comes back (3 minutes elapsed) 5-minute and 10-minute cross (13 minutes elapsed) 2-minute comes back (15 minutes elapsed) 1-minute and 2-minute cross (17 minutes elapsed)
93.39 %
38 votes

Similar riddles

See also best riddles or new riddles.


Two Japanese people

Two Japanese people who have never seen each other meet at the New York Japanese Embassy. They decide to have drinks together at a nearby bar. One of them is the father of the other one's son. How is this possible?
The Japanese are husband and wife and both blind since birth.
92.67 %
34 votes


Dead Man in the Desert

A man is found dead in the desert. He is wearing only his underwear. Half of a straw is found nearby. How did this man die?
The man was flying in a hot-air balloon with another man over the desert. The balloon started to go down because of excess weight. Both men would die if they ended up stranded in the desert, so they stripped down to their underwear and threw their clothes off the balloon to try to reduce the weight. Unfortunately, that didn't work well enough. So they drew straws to decide who would jump. The dead man pulled the short straw and jumped out of the balloon.
90.04 %
42 votes


One equals two

Consider the following explanation for why 1=2: 1. Start out Let y = x 2. Multiply through by x xy = x2 3. Subtract y2 from each side xy - y2 = x2 - y2 4. Factor each side y(x-y) = (x+y)(x-y) 5. Divide both sides by (x-y) y = x+y 6. Divide both sides by y y/y = x/y + y/y 7. And so... 1 = x/y + 1 8. Since x=y, x/y = 1 1 = 1 + 1 8. And so... 1 = 2 How is this possible?
Step 5 is invalid, because we are dividing by (x-y), and since x=y, we are thus dividing by 0. This is an invalid mathematical operation (division by 0), and so by not followinng basic mathematical rules, we are able to get strange results like these.
89.58 %
40 votes


Rectangular house

A man builds a house rectangular in shape. All sides have southern exposure. A big bear walks by, what color is the bear? Why?
White. The house is at the North Pole, so the polar bear.
92.86 %
35 votes


Predicting the score of the football game

Joe bets Tony $100 that he can predict the score of the football game before it starts. Tony agrees, but loses the bet. Why did Tony lose the bet?
Joe said the score would be 0-0 and he was right. "Before" any football game starts, the score is always 0-0.
93.39 %
38 votes


How far can you run

How far can you run into the woods?
Half way. If you go in any further, you’d be running OUT of the woods.
93.84 %
41 votes