= Solution
Assume individuals who immediately sell the intervention retain the untreated risk $0.005$, while those who keep it have the posited halved risk $0.0025$. The law of total <expectation> gives the expected count per thousand assigned to treatment:
$$
\boxed{200(0.005)+800(0.0025)=1+2=3.}
$$
The assignment risk is therefore $0.003$. This is an <intention-to-treat> mixture under the stated adherence-specific risks, not an assertion that receipt or sale is randomized. It also assumes that selling does not itself change the seller's baseline risk and that effects on recipients outside this group do not alter this calculation.
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