Brothers and sisters

There are 35 children in the class, 23 of them have a brother, and 27 of them have a sister. How many children have both a brother and a sister when there are 5 children in the class who have no brother or sister?

Correct result:

x =  20

Solution:

$$\begin{gathered} 30=x+27-x+23-x \\ x=23+27-(35-5)=20 \end{gathered}$$

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