Riddle #774

logic

3 Lightbulb switches

There are 3 switches outside of a room, all in the 'off' setting. One of them controls a lightbulb inside the room, the other two do nothing. You cannot see into the room, and once you open the door to the room, you cannot flip any of the switches any more. Before going into the room, how would you flip the switches in order to be able to tell which switch controls the light bulb?
Flip the first switch and keep it flipped for five minutes. Then unflip it, and flip the second switch. Go into the room. If the lightbulb is off but warm, the first switch controls it. If the light is on, the second switch controls it. If the light is off and cool, the third switch controls it.
94.11 %
43 votes

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logicmystery

Two Hourglasses

You have two sand hourglasses, one that measures exactly 4 minutes and one that measures exactly 7 minutes. You need to measure out exactly 2 minutes to boil an egg. Using only these two hourglasses, how can you measure out exactly 2 minutes to boil your egg?
Flip over both hourglasses at the same time. After 4 minutes, the 4-minute hourglass will be done, and there will be 3 minutes left in the 7-minute hourglass. Immediately flip the 4-minute hourglass over again. After 3 more minutes, the 7-minute hourglass will be done, and there will be exactly 1 minute left in the 4-minute hourglass. Immediately flip the 7-minute hourglass over again. After 1 more minute, the 4-minute hourglass will be done again, and there will be exactly 6 minutes left in the 7-minute hourglass. Immediately flip over the 4-minute hourglass. After 4 more minutes, the 4-minute hourglass will be done again, and there will be exactly 2 minutes left in the 7-minute hourglass. At this point, put your egg in the boiling water. When the 7-minute hourglass is done, it will have been exactly 2 more minutes, and your egg will have boiled just right.
84.78 %
55 votes

logic

Headwaiter in a restaurant

Betty signals to the headwaiter in a restaurant, and says, ''There is a fly in my tea.'' The waiter says ''No problem Madam. I will bring you a fresh cup of tea.'' A few minutes later Betty shouts, ''Get me the manager! This is the same cup of tea.'' How did she know? Hint: The tea is still hot.
Betty had already put sugar in her tea before sending it back. When the "new" cup came, it was already tasted sweet.
93.39 %
38 votes

logicmath

The missing dollar

Three people check into a hotel room. The bill is $30 so they each pay $10. After they go to the room, the hotel's cashier realizes that the bill should have only been $25. So he gives $5 to the bellhop and tells him to return the money to the guests. The bellhop notices that $5 can't be split evenly between the three guests, so he keeps $2 for himself and then gives the other $3 to the guests. Now the guests, with their dollars back, have each paid $9 for a total of $27. And the bellhop has pocketed $2. So there is $27 + $2 = $29 accounted for. But the guests originally paid $30. What happened to the other dollar?
This riddle is just an example of misdirection. It is actually nonsensical to add $27 + $2, because the $27 that has been paid includes the $2 the bellhop made. The correct math is to say that the guests paid $27, and the bellhop took $2, which, if given back to the guests, would bring them to their correct payment of $27 - $2 = $25.
93.39 %
38 votes

logic

Lost in the desert

Jack and Joe were on vacation and driving along a deserted country road from the town of Kaysville to the town of Lynnsville. They came to a multiple fork in the road. The sign post had been knocked down and they were faced with choosing one of five different directions. Since they had left their map at the last gas station and there was no one around to ask, how could Jack and Joe find their way to Lynnsville?
They need to stand the signpost up so that the arm reading Kaysville points in the direction of Kaysville, the town they had just come from. With one arm pointing the correct way, the other arms will also point in the right directions.
93.84 %
41 votes

logicmath

Flipping Lockers

There are 1 million closed school lockers in a row, labeled 1 through 1,000,000. You first go through and flip every locker open. Then you go through and flip every other locker (locker 2, 4, 6, etc...). When you're done, all the even-numbered lockers are closed. You then go through and flip every third locker (3, 6, 9, etc...). "Flipping" mean you open it if it's closed, and close it if it's open. For example, as you go through this time, you close locker 3 (because it was still open after the previous run through), but you open locker 6, since you had closed it in the previous run through. Then you go through and flip every fourth locker (4, 8, 12, etc...), then every fifth locker (5, 10, 15, etc...), then every sixth locker (6, 12, 18, etc...) and so on. At the end, you're going through and flipping every 999,998th locker (which is just locker 999,998), then every 999,999th locker (which is just locker 999,999), and finally, every 1,000,000th locker (which is just locker 1,000,000). At the end of this, is locker 1,000,000 open or closed?
Locker 1,000,000 will be open. If you think about it, the number of times that each locker is flipped is equal to the number of factors it has. For example, locker 12 has factors 1, 2, 3, 4, 6, and 12, and will thus be flipped 6 times (it will end be flipped when you flip every one, every 2nd, every 3rd, every 4th, every 6th, and every 12th locker). It will end up closed, since flipping an even number of times will return it to its starting position. You can see that if a locker number has an even number of factors, it will end up closed. If it has an odd number of factors, it will end up open. As it turns out, the only types of numbers that have an odd number of factors are squares. This is because factors come in pairs, and for squares, one of those pairs is the square root, which is duplicated and thus doesn't count twice as a factor. For example, 12's factors are 1 x 12, 2 x 6, and 3 x 4 (6 total factors). On the other hand, 16's factors are 1 x 16, 2 x 8, and 4 x 4 (5 total factors). So lockers 1, 4, 9, 16, 25, etc... will all be open. Since 1,000,000 is a square number (1000 x 1000), it will be open as well.
93.70 %
40 votes

logic

3 parachutes

The Pope, Beyonce, Barack Obama, and Bill Gates are on the same plane. There are only 3 parachutes left for the 4 of them. Obama says: "As the President, I think I should have the right to have a parachute, because I rule millions of people in the greatest nation of all." Beyonce says: "As one of the greatest singers of all-time, I think I should deserve to be safe. I bring tears and laughter to millions of people, and I'm an important contributor to pop music." Bill Gates says: "As one of the richest successful company owners, I think I should live because I'm on top of the economics cycle, creating jobs and incomes for millions of people. I am a wealthy and intelligent man." Finally, the Pope says: "I'm an old, religious man. I lived a life that's full, I helped millions of people find their way through God, I'm ready to let go of a parachute and to face my fate." Which one of them will abandon the parachute and die?
Did I ever mention that the plane was crashing? No one's gonna die.
92.14 %
54 votes

cleanfunnylogiclove

Love triangle

A doctor and a bus driver are both in love with the same woman, an attractive girl named Sarah. The bus driver had to go on a long bustrip that would last a week. Before he left, he gave Sarah seven apples. Why?
An apple a day keeps the doctor away!
83.71 %
83 votes