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 2005-09-04, 17:00 #1 fetofs     Aug 2005 Brazil 2·181 Posts Easy questions 1- A man buys bottles of wine for $1000. After taking 4 bottles and lifting the price of the dozen up$100, he sells it again for the same price. How many bottles did he buy? 2- How many 10cm diameter balls can you fit into a cubic box (1m dimensions)?
2005-09-05, 13:05   #2
fetofs

Aug 2005
Brazil

2×181 Posts

Quote:
 Originally Posted by fetofs 1- A man buys bottles of wine for $1000. After taking 4 bottles and lifting the price of the dozen up$100, he sells it again for the same price. How many bottles did he buy?
Looks like I was doing the wrong equation on this one... Considering b as the number of dozens of bottles and d as the dozen price...

$\Large bd=1000$

$\Large (12b-4)(d+100)=1000$

$\Large b=-\frac{5}{3}/2$

Guess it's the latter

2005-09-05, 13:34   #3
wblipp

"William"
May 2003
New Haven

3×787 Posts

Quote:
 Originally Posted by fetofs $\Large (12b-4)$
Thats the number of bottles, not the number of dozens of bottles.

 2005-09-05, 23:39 #4 Ken_g6     Jan 2005 Caught in a sieve 5×79 Posts 1. For some reason, I went at it the long way: p*n = 1000 n = 1000/p (p+100/12)*(1000/p-4) = 1000 (p+100/12)*(1000/p-4) = 1000 1000+100000/(p*12)-4*p-400/12 = 1000 100000/(p*12)-4*p-400/12 = 0 100000/12-4*p^2-400*p/12 = 0 -48*p^2-400*p+100000 = 0 p = (400+/-sqrt(160000+4*48*100000))/96 p = 4800/96 = \$50 per bottle So the answer is: n = 1000/50 = 20 bottles 2. First, the answer is >= 10*10*10 = 1000. Method 1: 1st bottom row: 10 across Now, the second row fits in spaces creating equilateral triangles. The centers of these balls are sqrt(3)/2 from the centers of the first row. Consider that at each end, half a sphere is required, but the rest can be filled in by triangles. Effectively, the first row takes 10cm, and the subsequent rows take ~8.66cm. However, every other row can only hold 9 balls. So: 90/8.66 ~= 10.39 > 10, so one can fit 11 rows. That's 6*10+5*9 = 105 balls on the bottom. If we stacked these vertically, one could get 10*105 = 1050 balls in the box. It is probably possible to do better, by nestling balls in subsequent layers in the triangles created by the layer below. Method 2: But first, let's place the balls in vertically equilateral triangles. Then the second layer takes ~8.66cm but each row must be the opposite size of the row below it. So this layer holds only 6*9+5*10 = 104 balls. We know this way we can get 11 layers, totaling 6*105+5*104 balls = 1150 balls. Method 3: Back to the balls in the triangles. Such a configuration of 4 balls forms a tetrahedron. According to Mathworld, the height of this [url=http://mathworld.wolfram.com/Tetrahedron.html]tetrahedron[/ a] is: 1/3*sqrt(6)*10cm ~= 8.165cm. So 90/8.165 ~= 11.02 > 11. So one can fit 12 layers in the box this way! Let's find out if we could fit one more row on the end of the second layer. The center of the balls on that last row would be offset by only a small amount. That amount is d on the Mathworld "bottom view" diagram. bad diagram: _ d|_\ 5 30 degrees d/5 = tan(30) => x = 5 tan 30 = 1/3*sqrt(3)*5 ~= 2.89cm. The space available ~= 3.9 cm, so it works! This means each layer will be just like the respective layer in case 2, only some will be shifted over by 2.89cm. 12 layers = 6*105+6*104 balls = 1254 balls. This has been proven to be the most efficient packing for an infinite size, but for a finite size there may be more efficient packings. I believe this is an open question. Last fiddled with by Ken_g6 on 2005-09-05 at 23:40 Reason: Removed column spacer
 2005-09-06, 00:17 #5 Mystwalker     Jul 2004 Potsdam, Germany 33F16 Posts Assuming the balls can be pressed into arbitrary shapes, we only have to take care of the volume. This is ½ d³ or 500cm³. Thus, 2000 balls (well, cubes by now :wink: ) would fit into the box... Last fiddled with by Mystwalker on 2005-09-06 at 00:19
2005-09-06, 13:00   #6
fetofs

Aug 2005
Brazil

2×181 Posts

Quote:
 Originally Posted by wblipp Thats the number of bottles, not the number of dozens of bottles.
Oh, forgot to add the $\Large (12b-4)/12$ Seems like I solved it correctly, and didn't input that part.

2005-09-06, 13:17   #7
fetofs

Aug 2005
Brazil

2×181 Posts

Quote:
 Originally Posted by Ken_g6 12 layers = 6*105+6*104 balls = 1254 balls.
That really wasn't what I expected to hear.
Quote:
 Originally Posted by Ken_g6 Consider that at each end, half a sphere is required, but the rest can be filled in by triangles. Effectively, the first row takes 10cm, and the subsequent rows take ~8.66cm.
I didn't understand how did you pack the spheres in equilateral triangles.... Maybe that way?
Code:
 O
OO
And how did you pack into tetrahedrons? A drawing would be nice, because I don't seem to understand the logic behind the smaller layers of triangles and polyhedrals...

Last fiddled with by fetofs on 2005-09-06 at 13:23

2005-09-07, 16:45   #9
fetofs

Aug 2005
Brazil

2·181 Posts

Quote:
 Originally Posted by wblipp Pythagorus tells us the third leg, which is the vertical distance from the center of layer 1 to the center of layer 2, is 10*sqrt(6)/3. about 8.165 cm. Packing in 12 layers will use up 11 inter-layer distances plus 5 cm on each end, totaling about 99.815 cm.

Greatly clarified! But how is 8.165*12+10=99.815

2005-09-07, 17:26   #10
wblipp

"William"
May 2003
New Haven

3·787 Posts

Quote:
 Originally Posted by fetofs Greatly clarified! But how is 8.165*12+10=99.815
As I said,

Quote:
 Originally Posted by wblipp Packing in 12 layers will use up 11 inter-layer distances plus 5 cm on each end, totaling about 99.815 cm.
Hence 8.165*11+10=99.815

2005-09-07, 21:53   #11
THILLIAR

Mar 2004
ARIZONA, USA

23 Posts
???

Quote:
 Originally Posted by wblipp This is easily calculated by starting with the equilateral triangle from 3 balls on the bottom, and observing that you can get from the center of any of these balls to the center of the upper ball by going to the centroid then up. Going straight up forms a right triangle, so we have a right triangle with the hypoteneus of 10 cm (2 radii) and one of the legs 10*sqrt(3)/3. Pythagorus tells us the third leg, which is the vertical distance from the center of layer 1 to the center of layer 2, is 10*sqrt(6)/3. about 8.165 cm. Packing in 12 layers will use up 11 inter-layer distances plus 5 cm on each end, totaling about 99.815 cm. This is tight. We could shrink this box to 100 x 99.49 x 99.82. I'd be surprised is anyone can wiggle enough to fit another ball.
This does not fly.

8.165 cm is less than the true inter-layer vertical distance.
Show us all your caculations to come up with that distance.

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