到底谁在说假话的推理问题：甲说：“乙说假话”，乙说：“丙说假话”，丙说：“甲乙二人都说假话”，问到底是谁在说假话？

数学题解答：这道理可以用假设法来想，如果甲说的“乙说假话”是真话，那么乙说的“丙说假话”是假话，即“丙说真话”，即“甲、乙两人都说真话”，这与一开始假设的甲说真话相矛盾，所以“乙说假话”是假话。

因为甲说假话，所以乙说的“丙说假话”是真话，即“甲乙二人都说假话”是假话。

综合以上分析：甲、丙两人都说假话，乙说真话。

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