Solution
= Solution
At month 36, patients F and D are censored at treatment durations 21 and 25, after which patient A is the sole remaining member of the <risk set> and dies at duration 26. The corresponding Kaplan–Meier factor is $1-1/1=0$, so
$$
\widehat F_{36}(48)=0
$$
without needing the earlier factors.
At month 48, D remains at risk past duration 26 and is eventually censored at 37, so no event empties the risk set. Part ii gives
$$
\widehat F_{48}(48)=\frac16>0.
$$