logic 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)

## Similar riddles

See also best riddles or new riddles.

logicshortWhat does this rebus say? XLR8.

Accelerate.

logicshortIn olden days you are a clever thief charged with treason against the king and sentenced to death.
But the king decides to be a little lenient and lets you choose your own way to die.
What way should you choose?
Remember, your're clever!

I would have chosen to die of "old age". Did you?

logicmathshortThe digits 0-9(0,1,2,3,4,5,6,7,8,9) can be rearranged into 3628800 distinct 10 digits numbers.
How many of these numbers are prime?

None. The sum of numbers from 0-9(0,1,2,3,4,5,6,7,8,9) is 45 and therefore can be divisible by 3 and 9.

logicMarty and Jill want to copy three 60 minute tapes. They have two tape recorders that will dub the tapes for them, so they can do two at a time. It takes 30 minutes for each side to complete; therefore in one hour two tapes will be done, and in another hour the third will be done. Jill says all three tapes can be made in 90 minutes. How?

Jill will rotate the three tapes. Let's call them tapes 1,2, and 3 with sides A and B. In the first 30 minutes they will tape 1A and 2A, in the second 3 minutes they will tape 1B and 3A (Tape 1 is now done). Finally, in the last 30 minutes, they will tape 2B and 3B.

logicYou are on your way to visit your Grandma, who lives at the end of the valley. It's her anniversary, and you want to give her the cakes you've made. Between your house and her house, you have to cross 5 bridges, and as it goes in the land of make believe, there is a troll under every bridge! Each troll, quite rightly, insists that you pay a troll toll. Before you can cross their bridge, you have to give them half of the cakes you are carrying, but as they are kind trolls, they each give you back a single cake.
How many cakes do you have to leave home with to make sure that you arrive at Grandma's with exactly 2 cakes?

2 Cakes
How?
At each bridge you are required to give half of your cakes, and you receive one back. Which leaves you with 2 cakes after every bridge.

logicshortTommy Tucker took two strings and tied two turtles to two tall trees. How many T’s in that?

There are 2 t’s in THAT.

logicmysteryscaryA crime has been committed at Freemont Street. The main suspect is a man named Sean Baker. It was said that a man had been walking along the pathway when he was suddenly shot in the stomach. The suspect had brown hair, blue eyes and wore a baggy Armani suit just like Sean Baker's. Sean was asked to tell the story right from the beginning. "Well," said Sean, "I was just hanging around the park when I saw this man walking along the pathway. Suddenly, a guy came up from behind him and shot him! I ran home as fast as I could. The policemen asked him to give a description of the murderer. "He had a red mustache, red hair and a baggy Armani suit on."
"I think this man is telling a lie," said one of the policemen. How did he know?

How can the murderer shoot him in the stomach if he came up behind the man?

logicshortWhat is the significance of the following: The year is 1978, thirty-four minutes past noon on May 6th.

The time and month/date/year are 12:34, 5/6/78.

logicIt was a very large truck. The truck need to cross a 3 mile long bridge. Unfortunately, the bridge can only hold the weight of 12000 lbs. Even a single pound extra, the bridge would collapse. However the weight of the truck is exactly 12000 lbs. The driver carefully drove and crossed almost 70 percent distance of the bridge. He stopped to get a small break. Suddenly, a bird landed on the truck. Did the bridge collapse? Justify your answers with explanation!

No. The bridge doesn’t collapse. The truck almost crossed 70 percent of total distance. Equivalent diesel would have been lost. So the extra weight of the bridge doesn’t add any extra load to the bridge.

funnylogicTwo men working at a construction site were up for a challenge, and they were pretty mad at each other.
Finally, at lunch break, they confronted one another.
One man, obviously stronger, said "See that wheelbarrow? I'm willin' to bet $100 (that's all I have in my wallet here) that you can't wheel something to that cone and back that I can't do twice as far. Do you have a bet?"
The other man, too dignified to decline, shook his hand, but he had a plan formulating.
He looked at the objects lying around: a pile of 400 bricks, a steel beam, the 10 men that had gathered around to watch, his pickup truck, a stack of ten bags of concrete mix, and then he finalized his plan.
"All right," he said, and revealed his object.
That night, the strong man went home thoroughly teased and $100 poorer.
What did the other man choose?

He looked the man right in the eye and said "get in."