Question 324693
FIRST TRY 12 & 12 SELECT THE HEAVIEST SIDE
SECOND TRY 6 & 6 DITTO
THIRD TRY 3 & 3 WEIGH 2 OF THE HEAVIEST SIDE.
IF ONE WEIGHS MORE - YOU"VE FOUND IT.
IF THEY MATCH - THE THIRD COIN IS THE HEAVIEST. 
THUS 4 TRIES ARE NEEDED.