75
votes
Born on the same day, but one is two years older
We'll just apply a bit of science assuming they were born in same year as well:
- 959
68
votes
Accepted
Measure 22 minutes with 7 and 13 minute hourglasses
A very simple solution, taking
is to
A better solution takes only
How?
- 27k
62
votes
Accepted
A clock correct three times a day
Since a stopped clock already agrees with a correctly running clock twice per day,
So the answer (that requires the smallest clock hand speed) is
Since the answer must be symmetrical (as long as the ...
- 69.3k
56
votes
50
votes
Accepted
47
votes
Accepted
46
votes
Born on the same day, but one is two years older
Apply some other bit of science, assuming they were born at the same time,
- 561
45
votes
Older twin's birthday is one day after younger's
Foo is older than Bar, but celebrates his birthday the day after Bar because one of the following happened:
- 15k
37
votes
32
votes
30
votes
A clock correct three times a day
Suppose we have an accurate clock and the fast clock, and that they start off showing the same time.
Here is an alternative method:
- 45k
29
votes
Accepted
28
votes
Accepted
A man is born in 1955
The answer is simply:
I have heard this one before but I don't remember where, maybe the movie you mentioned.
- 941
26
votes
Accepted
How often does a digital watch show a palindrome?
Let us denote the time by AB:CD:EF, where AB is between 00 and 23, CD and EF are each between 00 and 59, and of course A=F, B=E, C=D.
The middle two digits
Since ...
- 114k
24
votes
Accepted
All clock hands at equal degrees from each other
At noon, all three hands are at $0$ degrees from each other.
Let's say the Moment of Truth is after the hour hand has moved around $x$ degrees. In that time, the minute hand moves $12x$ degrees and ...
- 114k
24
votes
Measure 22 minutes with 7 and 13 minute hourglasses
22 minutes with lateral thinking (though the accepted answer is better)
- 943
24
votes
23
votes
Older twin's birthday is one day after younger's
Because they are...
And they were born...
And...
- 1,378
23
votes
22
votes
Accepted
Full-sized ice cubes
First produce 48 by putting the traces up/down/up/down/up in the freezer. Then use 18 of the ice cubes and put them in the corners of the botton 6 trays, produce 66 more ice cubes. Do this 2 more ...
- 6,874
22
votes
Accepted
21
votes
Accepted
Digital Clock Brightness
Logic behind the answer:
So, without really looking at anything else, my initial guess is:
resulting in:
Note:
Edit:
- 2,366
20
votes
Accepted
19
votes
Accepted
19
votes
Accepted
Using burning ropes as timer - how to measure 1 hour and 8 minutes with 3 ropes?
Ok I will present a solution.
For convenience, I will number the ropes in the order of their burning.
STEP ONE
STEP TWO
STEP THREE
FINAL STEP
I have successfully measured 1hour 8 minutes or 68 ...
- 354
19
votes
Accepted
Probability of three identical digits on a four-digit digital clock?
Number of qualifying times:
Probability:
- 10.2k
18
votes
Accepted
18
votes
Accepted
Riddle Man's age
The only perfect square in the 19th century is:
Which would make him
For the 20th century:
- 4,599
17
votes
Accepted
17
votes
Accepted
If the second hand goes backwards, starting from 12:00, how long does it take to meet with the minute hand?
The clockface is divided into $60$ segments.
- 45k
Only top scored, non community-wiki answers of a minimum length are eligible