Puzzles

Some good brain teasers are listed here.

Torch and 17 mins

Sent by Narayan of THBS on 11 Dec 2008 Thursday

4 people had to cross a delicate bridge in the night but had only one torch which can light for 17 mins.
Since the bridge is delicate, only 2 of them can cross the bridge at a time. The torch has to be brought back by one person for others to cross.
The 4 people have different speeds to cross.
A takes 1 min
B takes 2 mins
C takes 5 mins
D takes 10 mins.

If 2 people are crossing, the time required is the time taken by the person taking long time.
Ex. If A and D are crossing together, they require 10 mins.

How can all of them reach the other side of the bridge in 17 mins ?

Answer
[A,B,C,D] A,B goes first. A stays and B comes back. —> 2 mins as B accompanies.
Time is now 2 + 2 = 4 mins (forward and return journey by B)
[B,C,D] C,D goes next. Both C, D stays there and A now returns —> 10 as D accompanies. so 10 + 4 = 14 mins.
Now 14 + 1 (by A) = 15 minutes
[A,B] A, B goes and they reaches safely within 17 minutes. —> 2 as B accompanies [15 + 2 = 17 mins]

Why the man-hole cover is round in shape?

Sent by Narayan of THBS on 12 Dec 2008 Friday.

Did u observe Man-hole covers ( cover of the drainages ) ?. It will be in circular shape.
Why these are in circular shape but not in other shapes ?

Answer

Though I was almost closer to it, Ramakrushna of JPMC Team had contributed the answer (through google :)).


A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the aperture (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.)

Gift and Locks

Sent by Bhajan Mehta of THBS on 17 March 2009 Tuesday

You want to send a valuable object to a friend.
You have a box to contain the object.
The box has a locking ring which is more than large enough to have a lock attached.
You and your friend have several locks with keys.
But your friend does not have the key to any lock that you have, and vice versa.
How do you do it?
Note that you cannot send a key in an unlocked box, since it might be copied.

Answer:

U send the box to ur frnd with ur lock.
Ur frnd send the same box to u by putting his LOCK on it.
Then u remove ur lock (now it is having only ur frnd’s lock) and send to ur frnd.
Now ur frnd can open it at his end.
No one else can open the box in between…

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