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They all told 'yes' which mean either all of them are telling truth or all of them are lying. Probability of all of them telling the truth is 8/27 and probability of all of them are lying is 1/27 which means that out of all the 27 possible cases only 9 are applicable here. Out of those 9, 8 are of telling truth, so answer would be 8/9.

It's just my guess, i don't know if it is correct.