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First lets name the 8 coins,
A,B,C,D,E,F,G & H.

Now we will put A,B & C on one side of the weighing scale then D,E & F on the other side of the weighing scale. If the weighing scale is balance, then it shows that there is nothing wrong with coin A,B,C,D,E, & F, so the counterfeit coin would be either G or H if we do a weigh between this two coin.

But if the scale is not balanced when coin A,B,C and D,E,F is being compared, the coin on the lighter side of the scale would be taken out to find out which is the counterfeit coin.
EG. The total weight of A,B,C is lighter then the total weight of D,E,F, we will take coin A,B & C out. Then we will take any 2 coin to compare the weigh, if 1 of them is lighter that would be the counterfeit coin and if the scale is balanced, the last coin would be the counterfeit coin.