22 replies to this topic

[Ben Law]

You have 10 packets of cigarettes and each packet contain 10 cigarettes. We assume that the packets are weightless and that each cigarette weigh 10 grams. There is a packet within the lot where each cigarette weights 1 gram less i.e. 9 grams. What is the minimum times you must weigh to find out which packet is the one with the lesser weight cigarettes ???

[Jahudo]

Spoiler for answer

0...If we are allowed to open the packets instead of weighing them, then the packet with 9 cigarettes will weigh 9 grams.

[Jarod997]

Spoiler for First Guess

5 times - stick them on a balance two at a time. Once you find the pair which is not balanced you will know which one is the lighter one.

[firebirdaz]

Spoiler for guess

4 or 6
first weigh 5 and 5
take the 5 that weigh less and weigh two and two if they are equal you have found you light pack.
If they are not equal the wiegh the two that are lightest.

[Ben Law]

Spoiler for answer

0...If we are allowed to open the packets instead of weighing them, then the packet with 9 cigarettes will weigh 9 grams.

One packet had each cigareete weighing 9 grams each, so that pack weighed 9x10 = 90 grams instead of 100 grams

[sterlingmusk]

Spoiler for i got it..

3 or 2 if the second weighing equals out

5v5
2v2
1v1

[wh1zurd]

You have 10 packets of cigarettes and each packet contain 10 cigarettes. We assume that the packets are weightless and that each cigarette weigh 10 grams. There is a packet within the lot where each cigarette weights 1 gram less i.e. 9 grams. What is the minimum times you must weigh to find out which packet is the one with the lesser weight cigarettes ???

You need to weigh 4 or 6 times.
First 5 vs. 5 (2 times)
Lightest of this goes on
2 vs. 2 with one not being weighed (2 times)

If these are the same weight, you know that the packet that wasnt weighed is the lightest. If not, weigh the two packets and find which is the lightest (2 times)

[ccgolfballs]

Spoiler for Answer

Only 1 time if you use a scale! Take 1 from the first pack, 2 from the second, 3 from the third and so on for all ten packs. Weigh all 55 cigs at the same time(1+2+3..+10=55). If they were all 10 grams, it would wiegh 550 grams, but one pack was 1 gram off per cig. So if the total weight is 1 gram from 550, then it's the first pack, if 2 grams from 550, the second pack, 3grams from 550, the third and so on!

I got it

[initialj]

Spoiler for a diff answer

once.

stack the cigarettes up on the scale and take each pack off one at a time. you'll know if you grabbed the light pack when the scale reads a multiple of 10.

Edited by initialj, 03 July 2008 - 11:22 PM.

[akaslickster]

Spoiler for umb,

only once with or without scale put 5 in one hand and 5 in the other or use a balance thingy the pack will obviously be much lighter than the rest.

You have 10 packets of cigarettes and each packet contain 10 cigarettes. We assume that the packets are weightless and that each cigarette weigh 10 grams. There is a packet within the lot where each cigarette weights 1 gram less i.e. 9 grams. What is the minimum times you must weigh to find out which packet is the one with the lesser weight cigarettes ???