Using the empirical relationship: 3 Median = Mode + 2 Mean
$3 \text{ Median} = 15x + 2(18x) = 15x + 36x = 51x$
$\text{Median} = 17x$
Answer: (C) $17x$
The key formula (given as a Remark in Section 13.4) is 3 Median = Mode + 2 Mean. Substituting Mode = 15x and Mean = 18x gives Median = 17x. Examiners expect you to recall and apply this empirical relationship directly — no derivation needed for an MCQ.