Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 207 / 5 / b / ii / Solution

A pair is informative only when its first observed exit is an event while both members remain at risk; later events after one member has left contribute a factor one. The uninformative pairs are therefore pair 5, with tied events at $(17,17)$, and pairs 10, 11, and 13, whose earlier observations are censorings: $(63+,278)$, $(89+,1083+)$, and $(141,138+)$. At $(140,140+)$ both are in the risk set immediately before time 140, so pair 12 is informative.