i got it.... separate into 4 group 3(a)3(b)3(c)3(d), weight 2 of them(a,b), if it is the same, weigh b and c, if they are the same too... then the different ball must be in group d. if you weight different, you identify that group according to the different group. Also, we know the the different ball is heavier or lighter.
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, c2 s! a# v7 |& t" Nthey separate into 3 group111, weight 2 of them. if they are the same, the extra ball will be the different ball. if they are different, the heavier/ lighter ball will be the different one according to the previous weight. |