Đáp án đúng: D
Giải chi tiết:\(\begin{array}{l}\,\,\frac{3}{{x + 5}} = 15\% \\\frac{3}{{x + 5}} = \frac{{15}}{{100}}\\\frac{3}{{x + 5}} = \frac{3}{{20}}\\x + 5 = 20\\x\,\,\,\,\,\,\,\, = 20 - 5\,\,\,\,\\x\,\,\,\,\,\,\,\, = 15\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\end{array}\)
Chọn D