Please Help.... daughters homework driving me mad!

Obelisk29

Knew it would happen one day but was hoping not for a few more years!

If someone can shed some light on this by tomorrow morning I would be very appreciative:-

I have nine medals.

8 weigh 20g and 1 weighs 21g.

Using a scales (with no weights), how would you figure out the odd one out in just 2 weighs.

I can easily figure out how to do it in 3 goes - split in to 2 groups of 4, weigh, if they balance then the excluded medal is the odd one, if they don't balance, then take the heavier 4 and for the second weigh put 2 medals in to each pan. This will identify the group of 2 in which the 21g medal is in and then put one is each pan and the heavier is the one you're looking for.


but for the life of me I can't figure out how to do it in only 2 weighs.

Any clues?

o29

hallelujajordan

Split them into three groups of 3. Put two groups of 3 on the balance. If they balance take the third group and put one medal on each scale. If they balance the third is the 21g, if they don't the heavier is the 21g.

If your original 2 groups don't balance, take the heavier group of 3 and do the same thing.

2 weighs and you find the 21g !

Obelisk29

Bangs head of table.

Thanks - can sleep easily tonight.

hallelujajordan

Obelisk29 wrote:

Bangs head of table.

Thanks - can sleep easily tonight.

me 2 . . that wrecked my head for a good 10 minutes !