$$y=\frac{1}{x}\log x$$

(正答率 62%)

$$\begin{eqnarray*} y'&=& \left(\frac{1}{x}\right)'\times\log x +\frac{1}{x}\tim... ...log x+\frac{1}{x}\times\frac{1}{x}\\ &=& \frac{1-\log x}{x^2} \end{eqnarray*}$$