SOLUTION :6
Stage 1:
Seperate the coins into 3 stacks each with 9 coins (say A,B,C).
Measure stack A vs B (1) , if A=B then C will be the odd stack
else measure A vs C (2) if A=C then B is the odd stack.If A < or > C , A is the odd stack.
Thus in stage 1 we have done 2 measurements.
Stage 2:
Pick the odd stack and seperate into 3 more stacks each with 3 coins (say D,E,F).
Measure D vs E (3) , if D=E then F is the odd stack
else measure D vs F (4), if D=F then E is the odd stack. If D < or > F, then D is the odd stack.
Thus in stage 2 we have done 2 more measurements.Total 4.
Stage 3:
Pick the odd stack and let the 3 coins be G,H,I.
Measure G vs H (3) , if G=H then I is the odd stack
else measure G vs I (4), if G=I then H is the odd stack. If G < or > I, then G is the odd stack.
Thus in stage 3 we have done 2 more measurements.Total 6.
Therefore the answer is 6 number of weighings are required at minimum.
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