ข้อสอบคณิต O-NET - 2550

$$\begin{gathered} \mathrm{จากโจทย์} \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\mathrm{นำมาวาดรูป} \\ \mathrm{จะได้} \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}n\left(A\cup B\right) = 88 \dots \overline{)1} \end{gathered}$$

$n\left[\left(A-B\right)\cup \left(B-A\right)\right] = 76 \dots \overline{)2}$

$\mathrm{จะได้} \overline{)1} - \overline{)2} = 12$

$$\begin{gathered} \mathrm{จาก} n\left(A\right) = 45 \\ \mathrm{จะได้} n\left(A-B\right) = 45-12 \\ = 33 \\ \mathrm{จะได้} n\left(B-A\right) = 88-45 \\ = 43 \\ \mathrm{ดังนั้น} n\left(B\right) = 12+43 \\ = 55 \end{gathered}$$