,

$$\begin{eqnarray*} \sqrt{x+15}+5 & = & x\\ \sqrt{x+15} & = & x-5 \end{eqnarray*}$$

,

$$\begin{eqnarray*} x+15 & = & \left(x-5\right)^{2}\\ x+15 & = & x^{2}-10x+25\\ 0 & = & x^{2}-11x+10\\ 0 & = & \left(x-10\right)\left(x-1\right) \end{eqnarray*}$$

ซึ่งได้ $x=1$ และ $x=10$

,

,

$$\begin{eqnarray*} \frac{\log3x}{\log2}+\frac{\log9x}{\log2^{2}}+\frac{\log27x}{\log2^{3}} & = & 3\left(\frac{\log2}{\log2}\right)+2\left(\frac{\log x}{\log2^{6}}\right)\\ \frac{\log3x}{\log2}+\frac{\log9x}{2\log2}+\frac{\log27x}{3\log2} & = & 3\left(\frac{\log2}{\log2}\right)+2\left(\frac{\log x}{6\log2}\right)\\ \log3x+\frac{1}{2}\log9x+\frac{1}{3}\log27x & = & 3\log2+\frac{1}{3}\log x \end{eqnarray*}$$

,

$$\begin{eqnarray*} \log\left(3x\right)+\log\left(9x\right)^{\frac{1}{2}}+\log\left(27x\right)^{\frac{1}{3}} & = & \log8+\log x^{\frac{1}{3}}\\ \log\left[\left(3x\right)\left(9x\right)^{\frac{1}{2}}\left(27x\right)^{\frac{1}{3}}\right] & = & \log\left[8x^{\frac{1}{3}}\right]\\ \log\left[27x^{1+\frac{1}{2}+\frac{1}{3}}\right] & = & \log\left[8x^{\frac{1}{3}}\right]\\ 27x^{1+\frac{1}{2}+\frac{1}{3}} & = & 8x^{\frac{1}{3}} \end{eqnarray*}$$

,

$$\begin{eqnarray*} x^{1+\frac{1}{2}+\frac{1}{3}-\frac{1}{3}} & = & \frac{8}{27}\\ x^{\frac{3}{2}} & = & \left(\frac{2}{3}\right)^{3}\\ x^{\frac{1}{2}} & = & \frac{2}{3}\\ x & = & \frac{4}{9} \end{eqnarray*}$$

ดังนั้น $B=\left\{\frac49\right\}$

,