7.1. People stand in a circle - liars who always lie, and ryiars who always tell the truth. And each of them said that among the people standing next to them, there are as many liars as ryiars. How many people are there in total if there are 48 ryiars?
Answer: 72

Solution. Let's denote a knight as $\mathrm{P}$, and a liar as L. Notice that each knight stands between a knight and a liar, otherwise he would have told a lie. Therefore, knights stand in groups of two. Liars cannot stand in groups larger than one, as in such a case, the liar standing at the edge of the group would have told the truth. Thus, groups of two knights and one liar alternate: ... PPPLPPPLPPP... Since there are 48 knights, there are 24 liars. Therefore, there are 72 people in total.