Questions tagged [time]

Puzzles which include the concept of time, as for instance: clocks, dates, calendars, conversions from one time zone to another, time-intervals.

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53
votes
12answers
11k views

A clock correct three times a day

A stopped clock tells the right time twice a day. How fast must a clock go for it to be right three times a day?
39
votes
5answers
5k views

Burning ropes as timers

You have 2 identical ropes which burn at a specific rate, and an unlimited supply of matches. When you light one end of a rope, the fire will take exactly 1 hour to travel to the other end of the rope....
35
votes
5answers
9k views

Measure 22 minutes with 7 and 13 minute hourglasses

You have two hourglass timers. One can measure 7 minutes and the other hourglass timer can measure 13 minutes. This means that it takes 7 minutes for the sand timer to completely empty the sand from ...
32
votes
16answers
11k views

Older twin's birthday is one day after younger's

I have been asked this puzzle in an interview. There was not a lot of time left so I couldn't solve it quickly and the interviewer kind of let it go as he had to wrap up the interview Foo and Bar ...
30
votes
11answers
13k views

Born on the same day, but one is two years older

Two people were born on the same day, same year, and share the same birthday. One is 2 years old, the other is only a few days old. What's going on here?
23
votes
3answers
2k views

Digital Clock Brightness

My alarm clock has a typical 7-segment digital display showing hours and minutes. Every lit segment is equally bright, and every unlit segment is equally dark. The separator (:) is always lit. ...
19
votes
1answer
1k views

Aging A Lot Faster

The day before yesterday, I was 20 years old, and next year I will turn 23. What day is it today? Note: This is not lateral-thinking, and the solution is real and ...
17
votes
14answers
11k views

My birthday is uncommon

I was born 1 day after my sibling. Yesterday was my sibling's birthday but today is not my birthday. When is my birthday?
17
votes
6answers
4k views

A weird and spooky clock

Out of nowhere, a clock appeared on my desktop: (you don't have to be able to view the GIF in order to solve it, I think the "preview" image gives you all the minor clues hidden in it. I also ...
17
votes
3answers
2k views

Two brothers, two watches

Two brothers, Jack and Joe, were born in May. One was born in 1932, and the other, a year later. Each brother owned a watch, which was previously possessed by their father. Considering their age, ...
16
votes
1answer
2k views

Finally, you wake up from a coma

You can barely open your eyes when a doctor walks up to you: You are a lucky one. Not many people have survived an accident like this. Only a handful of neurosurgeons are able to perform such an ...
15
votes
2answers
489 views

“Musical chair” for clock hands

A colleague and me usually make bets on the time when a simulation is finished. Next time I would suggest the following: At the time the simulation is finished we take a look at the analog clock on ...
13
votes
5answers
3k views

How often does a digital watch show a palindrome?

A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward or forward, such as madam or kayak. We have a digital watch showing the time HH:MM:SS as shown in ...
13
votes
3answers
846 views

One More Twin Puzzle: Four Days Difference?

This is updated version for Another Twin Puzzle: Three Days Difference Today is my first day joining my college's Puzzle Club. It is so much fun gathering in a group with similar interests! I meet a ...
13
votes
1answer
1k views

Visiting the gym this month

Question Three men visit a gym on a regular basis - one every two days, one every three days, and the last one visits once every week (on the same day). In a particular month, one of the 3 men's ...
12
votes
3answers
351 views

You cannot have 120 degrees of separation, but how close can you get?

If you take a look at a standard three-hand gear-driven analog clock, there is never(see footnote) a time of day where the hands of the clock divide the face equally into 120 degree segments, but you ...
12
votes
3answers
817 views

Day and night of the two timers

            Make a non - 24 - hour day /  night cycle       &...
11
votes
5answers
3k views

This puzzle is late!

I meant to release this puzzle 15¼ hours ago, and forgot again just over 9⅓ hours ago. But don't worry, it's fine. The puzzle is finally out. What is this puzzle about? NB: What timezone I am ...
11
votes
2answers
2k views

When was I born?

I was born just 10 minutes apart from my twin brother, but I celebrated my birthday 11 days after my twin brother every year. When was I born ?
11
votes
4answers
619 views

Short Puzzle: Graphic Painting

The artist in the market is giving this painting for free (the last one on his rack). I asked him if I can take it but he replied "Why should I give it to you?" The frame could be useful but I dont ...
11
votes
2answers
450 views

Another Twin Puzzle: Three Days Difference

Today is my first day joining my college's Puzzle Club. It is so much fun gathering in a group with similar interests! I meet a lot of new friends and of course asked their contact number to discuss ...
10
votes
4answers
817 views

How to rephrase this puzzle about a meditating monk and incense

Here's a math-flavored puzzle I found on Andrej Cherkaev's website that I like to give to my undergraduate math students. I've paraphrased it to try to make it clearer: A monk was instructed by his ...
10
votes
4answers
636 views

Mirrored clocks

Triangulating for the simplest puzzle that is still at least somewhat interesting to solve.. On the left side wall in this picture, we have two particular clocks: 1: an analog clock with identical ...
9
votes
6answers
2k views

A mosquito is born at T=0 and after 2 seconds makes a new mosquito every second, how many mosquitos at T=8.5? [closed]

One mosquito is born at T=0 and after 2 seconds it makes a new mosquito, and another every second thereafter. All such mosquitoes have this same property and do not die. How many mosquitoes there are ...
9
votes
3answers
2k views

Missed appointment

The story This story occurs not far from Liège, in Belgium. We are January 1st, 2013. Two bad men prepare the attack of a train. To do this, they set an appointment, during the year, at 2:30 a.m., ...
9
votes
2answers
484 views

A Final Twins Puzzle: Twelve Days Difference?

Inspired by @athin's One More Twin Puzzle: Four Days Difference? After reading @athin's question, I was inspired to ask my own puzzle club about their birthdays. One of the members, Ethan, surprised ...
9
votes
2answers
271 views

Where are you at this time

Where are you if, When it's 10 o'clock, it's 2 o'clock When it's 5 o'clock, it's 7 o'clock
9
votes
2answers
361 views

Third timer's a charm

Three motorized 24-hour light timers are arranged between a power outlet and a light bulb, with timer A parallel to a daisy chain of timers B and  C. For these timers, devise schedules that ...
8
votes
3answers
540 views

Halve time with two timers

Two motorized 24-hour light timers are daisy chained between a power outlet and a light bulb. For these timers, devise schedules and choose initial times that produce the following repeated lighting ...
7
votes
5answers
710 views

One a puzzle, two a puzzle, Hot Cross Clocks

It's Good Friday! I want to make perfect hot cross buns by superimposing two 12-hour clock faces on top of each other. Is this possible, and if so, how many ways are there of doing so? In other ...
7
votes
3answers
737 views

Odd hours with two timers

Two motorized 24-hour light timers are daisy chained between a power outlet and a light bulb. For these timers, devise schedules and choose initial times that produce the following repeated 9-hour ...
7
votes
5answers
16k views

Meeting of hands on a clock

We have a regular analog clock with three hands - second, minute and hour. On a given day, how many times do: (a) the minute and hour hands (b) the minute and second hands (c) the hour and second ...
7
votes
2answers
331 views

How many dates are ambiguous across the Atlantic?

The Day/Month/Year date system commonly used in Britain (and much of Europe?) and the Month/Day/Year system commonly used in the US can lead to confusion when, for example, we write 9/3/2018 for the ...
7
votes
1answer
251 views

What is it this Time?

Most clocks tell the time, but some are for letters. ...
6
votes
7answers
17k views

A man is born in 1955

(I heard this in a movie and I think it’s pretty cool.) A man is born in 1955. Today, the 14th of August, 2017, he is 18. How is this possible?
6
votes
7answers
3k views

Full-sized ice cubes

On a table in front of you is a small freezer that is capable of holding seven standard ice cube trays stacked on top of each other. There are no shelves to separate the trays, and if you stack one ...
6
votes
6answers
3k views

Timepieces and Moving Parts

The sundial keeps time with the fewest number of moving parts. Which timepiece has the most moving parts?
6
votes
3answers
698 views

Ages of A and B (self-reference in past)

A says to B My age is twice of what your age was when I was of your age. B says to A If you add your current age to twice of your age when I was born, the answer is $63$. Assuming they both ...
6
votes
2answers
1k views

What's Bob's age again?

I created this problem myself, but I did have inspiration. Today is a special day, I'm moving out of my parent's house. My father comes to me and says "Wow, you're half my age. On the day we moved ...
5
votes
1answer
918 views

A time-traveller's puzzle

Intense and long story coming up... In a galaxy, far, far away there lived a population of creatures, human-like but definitely smarter. The whole galaxy experienced three phases for every Earth ...
4
votes
2answers
760 views

What timezone am I?

It was 17:52 in the evening and I couldn’t wait for my class to finish. Only 8 more minutes… I really hate night school! I sat there pondering the stuff on the blackboard and wondering why he wrote ...
4
votes
3answers
207 views

A Second Hand Question

" It is a very simple question", said Grandpa " Look at that wall clock and tell me exactly how many times the Second Hand passes directly over the Minute and the Hour Hands in a 24 hour period....
4
votes
2answers
232 views

Switching the clock

"Observe the time, would you, Watson, old chap?" I did as instructed and glanced at the large grandfather clock standing in the corner. "Just a little after 4.42, Holmes." He nodded, "So when the ...
4
votes
4answers
2k views

An Hourglass Puzzle

This puzzle follows from the well known sand timer puzzles: A Sand Timer Challenge http://www.crazyforcode.com/sand-timer-puzzle/ http://www.cut-the-knot.org/hg_solution.shtml Warmup: You are given a ...
4
votes
5answers
430 views

Time-Motion puzzle

Time and Motion studies are well established. Here is an easy question: It takes 8 men 6 months to build 2 houses at a cost of $50,000. It takes 10 men 5 months to build 3 houses at a cost of ...
4
votes
3answers
532 views

The time is approximately 10:10

Watches in advertisements tend to be set to 10:10 (unless you're Apple). The reason seems to be that the hands are symmetrical when they're at 10:10. Except they're not. The hour hand moves ...
4
votes
1answer
490 views

Connecting 3 circular power timers to lights so that they turn for one day on Christmas

My friend gave a question to me. I am not sure if it is an original puzzle, but he has 2 lines of Christmas lights and 3 circular timers with 96 switches apiece. Each switch turns on power for 15 ...
3
votes
8answers
7k views

How many minutes is it before 12 noon? (tricky)

This puzzle is a one I met in an IQ test. I believe it's so tricky, however, the concept is quite simple. How many minutes is it before 12 noon if 68 minutes ago it was three times as many minutes ...
3
votes
2answers
495 views

Your age and my age [closed]

This is not a very difficult problem, but it seems that it hasn't been asked here. It's one of my favorites, so here we go: I am twice as old as you were when I was as old as you are. When you ...
3
votes
5answers
588 views

Curiosity of February 5$^{th}$

Some people write February 5$^{th}$ using the md format, as $2.5$ or alternatively as $2/5$. Note that $2.5 \times \frac25 = 1$. Is there any other date satisfying $a.b \times \frac{a}{b} = 1$? ...