# 5. Option 1.

Café "Buratino" operates 6 days a week with a day off on Mondays. Kolya made two statements: "from April 1 to April 20, the café was open for 18 days" and "from April 10 to April 30, the café was also open for 18 days." It is known that he was wrong once. How many days was the café open from April 1 to April 27?
Answer: 23

Solution: In the period from April 10 to April 30, there are exactly 21 days. Dividing this period into three weeks: from April 10 to April 16, from April 17 to April 23, and from April 24 to April 30, we get exactly one weekend (Monday) in each of the three weeks. Therefore, in the second statement, Kolya told the truth, and in the first statement, he was wrong.

The period from April 1 to April 20 can be divided into two weeks: from April 1 to April 7, and from April 8 to April 14, and a six-day period (from April 15 to April 20). In the two weeks, there are 12 working days, so in the period from April 15 to April 20, there could be either six working days or five. Since Kolya was wrong here, there were five. That is, in the period from April 15 to April 20, there was definitely a weekend, as well as in the period from April 22 to April 27 (since it is obtained by shifting the previous period exactly by 7 days).

In total, from April 1 to April 21, the cafe was open $3 \cdot 6=18$ days, and in the period from April 22 to April 27, another $6-1=5$ days, that is, a total of $18+5=23$ days.