Question

Poly, Mili and Lily are three sisters. Lily tells truth in 6 days of a week. Poly tells
truth in one day per week and Mili tells truth in 2 days per week. If Poly tells truth
on a day, Mili also tells truth on that day. At Friday, Lily told Mili, “Today Poly
will tell truth.” But Mili said, “No. Tomorrow Poly will tell truth.” Then Poly said,
“Lily is right.”. On which day of week, Poly tells truth?

Answer 1

Poly, Mili and Lily are three sisters. Lily tells truth in 6 days of a week. Poly tells truth in one day per week and Mili tells truth in 2 days per week. If Poly tells truth on a day, Mili also tells truth on that day. At Friday, Lily told Mili, "Today Poly will tell truth." But Mili said, "No. Tomorrow Poly will tell truth." Then Poly said, "Lily is right." On which day of week, Poly tells truth?

**Solution.**

If Poly tells truth today, then Mili also tells truth: "No. Tomorrow Poly will tell truth." It follows from this that Poly tells lie today. We got a contradiction.

Consider another case. If Poly tells lie, then Lily is wrong: "Today Poly will tell truth." If follows from this that today Poly won't tell truth. So tomorrow Poly will tell truth.

**Answer:** Poly tells truth on Saturday.