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Breaking the Billiard Ball

Logic puzzles require you to think. You will have to be logical in your reasoning.

 

Puzzle ID:#8878
Fun:*** (2.81)
Difficulty:*** (2.93)
Category:Logic
Submitted By:lesternoronha1****
Corrected By:phrebh

 

 

 



It is your task to determine how high you can drop a billiard ball without it breaking. There is a 100 story building and you must determine which is the highest floor you can drop a ball from without it breaking. You have only two billiard balls to use as test objects. If both of them break before you determine the answer then you have failed at your task. What is the order of floors that you should drop the balls from to minimize the number of droppings that you will have to make to determine the answer?

Assume that if a ball doesn't break you can reuse it without worrying about it being weakened.




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Comments

LimnShicks2
Dec 17, 2002

I don't know how someone can start with 14 and start from there. This was a very confusing and, yet, hard to guess problem.
dewtell*us
Jan 04, 2003

Try working backwards from the top (there
are several slightly different solutions).
E.g., if you still have 2 balls left and
have 5 floors to cover, you can do that in
three tosses by starting from floor 98.
Whenever you break the first ball, you will have
to cover whatever remaining floors are between
it and the last successful drop by working
up with the remaining ball one floor at a time.
So if the last successful ball #1 drop was from
floor #95 (to leave 5 floors/3 moves left if it succeeds),
the previous drop must have been from floor
91 or higher, so that there are only 3 moves left
if the drop from #95 breaks instead of works.
Hence the previous drop was from #91, etc.
na-iem
Mar 12, 2003

Aah! I see! Effectively we need minimum n such that:
1 + 2 + 3 + ... + n >= 100

Neat :-)
Codammanus
Jun 18, 2003

This isn't logic, its math.
JessicagAau*
Aug 14, 2003

why not start from floor one?
If it doesn't break then go up to floor 2, if it doesn't break then go up to floor 3 and so on. If it breaks on floor x, you hav egone one level too high so you go back down one and drop unbroken billiard balls to your hearts delight. Perhaps I'm missing something crucial because this seems too easy.
flynn
Aug 14, 2003

Jessicag, I also figured your could start at floor one and work your way up. But the puzzle was to do it in the least # of steps. Doing it out way might be OK if the answer was less than 14--but if it was floor 99 then that certainly is not the least number of steps.

dfitzp
Aug 14, 2003

Nice one - but I have problem with the solution - given the rules laid down. It relies on both balls breaking at some point to determine the answer but the rules state that you have failed if both balls break before the answer is determined
dewtell*us
Aug 16, 2003

Not quite - the rules say that if both balls
break *before* you have determined the
answer, you fail. In the solutions
given, you know the answer, at the latest,
the instant the second ball breaks. In
some cases, the second ball never breaks.
sakirski*
Aug 20, 2003

Just wanted to respond to the comment that this isn't a logic but rather a math problem.
It may contain elements of basic math, but those are trivial. The solution can best be developed through logical reasoning.
coastcomfort
Aug 15, 2004

If someone really wants to be a wise guy they could figure out the terminal velocity of the billiard ball, by knowing its mass and air resistance. Factor in your 9.8m/s/s for gravity to determine the height at which that velocity is reached and deduct those floors. I'm too lazy to do the math, so TV might not be reached within 100 floors, blowing all this out of the water. Great teaser though.
anotherMe
Dec 18, 2004

One of the best ones I have seen on this webpage... keep it up!
bfriedfischtom
Apr 17, 2008

There is a slightly better solution that leaves the max at 14 but has a better average.
The principal is exactly the same but we use the fact that with the 11 drop of the first ball we are already at 99.
The new solution follows.
The columns are
#step/floor-difference/floor
01-13-013
02-12-025
03-11-036
04-10-046
05-09-055
06-09-064
07-08-072
08-07-079
09-06-085
10-05-090
11-04-094
12-03-097
13-02-099
14-01-100
the difference is that we start with 13 (instead of 14) but repeat the 9-floor step.
Thus below floor 55 we only need 13 max, above it is still 14.
This however gives a lower average.
I am not sure it this is the solution with the lowest average.
bfriedfischtom
Apr 17, 2008

picky:
the average drops from 9.48 to 9.45 (wow, that is 945 drops instead of 948 if we cover all possiblities).



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