I have two sets of 8 coins. In one set the coins weigh 30g each, in the other set the coins weigh 31g each.
Unfortunately they got mixed together in one big pile of 16 coins. I want to identify one coin. It can be from either set.

(Easy) Using a balance scale, identify a coin in four weighings.

(Hard) Identify a coin in just three weighings.

(In reply to Solution (hard) by Paul)

It's more interesting if 1+2 = 3+4. In this case as the 2nd weighing, compare 1+2+3+4 to 5+6+7+8, knowing that all four of the left coins are identical (but not whether they're light or heavy.)

WRONG    what if  both  1+2   and 3+4  weigh 61 g

 the rest is wrong as well

Posted by Ady TZIDON on 2015-12-30 17:01:14