[By popular request, here are the questions to the answers in this post.]

1. Why are manhole covers round?

2. There are three switches in one room and one light bulb in another. How can you tell which switch controls the bulb if you can only make one trip from the switch room to the bulb room?

3. If a plane crashes right on the border of the US and Canada, in which country would you bury the survivors?

4. You have two fuses which burn unevenly (not at a constant rate). You do know that it takes one minute for each fuse to burn completely when lit at one end. How do you measure 45 seconds?

5. You have four people (A, B, C & D) who have to cross a bridge. One or two can cross at once and they travel at the speed of the slower person. There's one flashlight which is needed to cross the bridge. A takes one minute, B takes two minutes, C takes five minutes and D takes ten minutes to cross. How do you get all of the people across in 17 minutes?

6. How far apart are the hands of a clock at 3:15?

And the ever popular:

7. Tell me one of your faults.

Anonymous said…
i know n.4 with 15 seconds, which is generally the same problem.

now, a couple of years ago, an assistant professor in mathematical analysis after 10 min think gave me an answer for the 15 second version, using only one fuse. :)
Anonymous said…
foo said...
now, a couple of years ago, an assistant professor in mathematical analysis after 10 min think gave me an answer for the 15 second version, using only one fuse. :)

Anonymous said…
Answer 6: Assuming we are talking about the hour and minute hand.

7.5 degrees

Explanation: minute hand is at the 15 minute mark. Hour hand is 1 quarter of the way between 3 and 4. So the question breaks-down to how many degrees is 1 quarter of the distance between a single hour. There are 360 degrees on the clock. Divide 360 degrees by 12 hours, which gives you 20 degrees between each hour. So 1/4 x 30 degrees equals 7.5 degrees.
Anonymous said…
So turn one switch on for a minute then turn it off. Turn another switch on and leave it on. Go check the bulb. If its on its the second switch, if its off and hot its the first and if its off and cold its the third. Though LEDs might put paid to this.
Emmanuel said…
Just a quick note.

Problem 2 is only valid if we know the light bulb is off at the beginning of the test.
Anonymous said…
7.5 degree is no measure of distance.
Unknown said…
@anonymous You're correct! Degrees are'nt a measure of distance ("How far apart are the hands"), but radians are. (Read the BetterExplained Article here.)

So 7.5° is a full circle by 48 (360°/15°). Now a full circle is equal to 2π (The circumference of circle -- 2πr -- with r equal to one); So, in radians, the answer would be 2π/48 = π/24 ≈ 0.130899694.
Unknown said…
@anonymous You're correct! Degrees are'nt a measure of distance ("How far apart are the hands"), but radians are. (Read the BetterExplained Article here.)

So 7.5° is a full circle by 48 (360°/15°). Now a full circle is equal to 2π (The circumference of circle -- 2πr -- with r equal to one); So, in radians, the answer would be 2π/48 = π/24 ≈ 0.130899694.
Anonymous said…
Radians aren't a measure of distance.
Anonymous said…
Lee said...
foo said...
now, a couple of years ago, an assistant professor in mathematical analysis after 10 min think gave me an answer for the 15 second version, using only one fuse. :)