You have 39 balls in front of you. All but one are of equal weight. You do not know whether the defective ball is lighter or heavier than the normal balls. You are given a comparison balance. You can use the balance only 4 times. How would you find out which is the defective ball? Is it heavier or lighter?
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Answer:
compare 19 v 19, if equal, then one left out is it, if not, take one of lesser weight
compare 9 v 9, if equal, then one left out is it, if not, take one of lesser weight
compare 3 v 3, if equal, compare 1v1 of remainder, if equal, then remainder 1 is it, if not, lesser is it,
if 3v3 is less, compare 1v1 of remainder, if equal, then remainder 1 is it, if not, lesser is it,
