= Solution
On the stipulated figures, a 20% decline from 200 <cocaine>-related deaths means about 40 fewer deaths, against about 10 attributed to <mephedrone>:
$$
\boxed{40-10\simeq30\text{ fewer deaths per year in this restricted comparison}.}
$$
That bookkeeping is relevant to a population-harm assessment only if the avoided deaths really result from substitution, the new deaths are counted comparably, and overlapping drug attributions are not counted twice. A change in the legality of one drug is an intervention affecting behaviour; the before-and-after association alone does not identify its <causal effect>.
Distinguish per-user danger from total population harm. More use can raise total deaths even for a less dangerous drug, and an increase can involve new users who would otherwise use no drug. Conversely, restricting a substitute may move users back to more dangerous alternatives. The relative fatality risks do not mechanically determine legal classification: nonfatal harm, dependence, dosage and purity, co-use, the composition of users, enforcement and supply effects, and uncertainty in the estimates all matter. Any policy assessment should compare plausible behavioural responses to the alternatives, including displacement of both <cocaine> and <MDMA>, rather than extrapolate from ten deaths alone.
\b[The hypothetical net reduction is a reason to examine harm reduction and substitution carefully, not sufficient evidence for a categorical legal recommendation.]
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