= Solution
Expected death counts are the number of users multiplied by the death risk per user, with matching time windows and definitions. Relative to the <MDMA> benchmark, the proposed use prevalence contributes a multiplier of $0.6$ and the per-user fatality risk contributes a multiplier of $0.5$. Hence
$$
\boxed{30\times0.6\times0.5=9\text{ mephedrone-only deaths}.}
$$
This assumes comparable user exposure durations and recording of single-drug deaths. Population death counts alone would not identify a per-user <risk ratio> without the prevalence denominator.
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