Village flag waving

Sixty children from the school accompanied them to celebrate the village's founding. Forty children waved with the village flag. Thirty children got waved with the coat of arms of the village. How many children received both types of lubricants?

Final Answer:

x =  10

Step-by-step explanation:

$$\begin{gathered} n=60 \\ A=40 \\ B=30 \\ x=A\cap B \\ n=A+B-x \\ x=A+B-n=40+30-60=10 \end{gathered}$$

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