[Text problems (other) ]

At each of the four math club sessions, 20 schoolchildren were present. Nine students attended exactly three out of these four sessions, five students - exactly two sessions, and three students only attended one session. How many schoolchildren attended all sessions?
Let's fill in the "attendance record" for this club. In total, for the specified four sessions, there will be $20 \cdot 4=80$ attendance marks. Each student who attended three sessions is marked three times in the record, so nine such students are marked a total of $9 \cdot 3=27$ times. Similarly, those who attended twice are marked a total of $5 \cdot 2=10$ times. Three students who attended once are marked a total of three times in the record. The remaining $80-(27+10+3)=40$ attendance marks are for those who attended all four sessions, with each of them marked four times. Therefore, the number of students who attended all sessions is $40: 4=10$.

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