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L _ G H T _ R Clue
STEP 1: DIVIDE IT INTO SETS OF THREE. IF YOU PUT ONE SET ON EITHER SIDE, IT'LL EITHER EQUAL THE SAME, OR YOU'LL SEE WHICH SET IS LIGHTER. EITHER WAY, YOU NARROW IT DOWN TO THREE BALLS. STEP 2: PUT TWO OF THE BALLS ON THE SCALE. IF THEY'RE EQUAL, THE LAST BALL IS DEFECTIVE, BUT IF NOT, THE SCALE WILL TELL YOU WHICH ONE IS LIGHTER A MINIMUM OF 3 WEIGHINGS IS REQUIRED. DIVIDE THE 27 BALLS INTO 3 GROUPS OF 9 BALLS EACH. USE THE SCALE TO WEIGH 2 GROUPS. THIS WILL TELL US WHICH GROUP HAS THE HEAVIER BALL. NOW, DIVIDE THE 9 BALLS INTO 3 GROUPS OF 3 BALLS EACH AND WEIGH 2 GROUPS, AND IDENTIFY THE GROUP WHICH HAS THE HEAVIER BALL. FROM THE HEAVIER GROUP, WEIGH 2 BALLS TO DETERMINE THE HEAVIER BALL SPLIT THE COINS INTO 3 GROUPS - 2 GROUPS WITH 3 COINS EACH AND 1 GROUP WITH 2 COINS. NOW PUT THE 2 GROUPS OF 3 COINS ON THE SCALE. SCENARIO #1: BOTH GROUPS WEIGH THE SAME. THIS MEANS THAT THE COUNTERFEIT COIN IS IN THE GROUP WITH 2 COINS. SO TAKE THE 2 COINS FROM THAT GROUP AND USE THE SCALE TO DETERMINE THE COUNTERFEIT COIN. SCENARIO #2: ONE GROUP WEIGHS LESS THAN THE OTHER. TAKE ANY 2 COINS FROM THIS GROUP OF 3 COINS. IF THEY WEIGH THE SAME, THEN THE 3RD COIN IS THE COUNTERFEIT COIN. IF THEY DON'T WEIGH THE SAME, THEN THE LIGHTER ONE IS OBVIOUSLY THE COUNTERFEIT COIN TAKE 6 BALLS AND SPLIT THEM INTO 2 GROUPS OF 3 BALLS EACH. USE THE SCALE TO WEIGH THE 2 GROUPS. IF ONE OF THE GROUPS IS HEAVIER THAN THE OTHER GROUP, PICK 2 BALLS FROM THE HEAVIER GROUP AND USE THE SCALE TO WEIGH THEM. IF THEY ARE EQUAL IN WEIGHT, THE OTHER BALL IN THAT GROUP IS THE HEAVY ONE. IF BOTH THE GROUPS ARE EQUAL IN WEIGHT, THEN PICK THE REMAINING 2 BALLS AND USE THE SCALE TO DETERMINE THE HEAVIER BALL