 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 $1-1/1=0$, sowithout 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