= Solution
Use time since enrolment, and assume the cohorts share a common <survival distribution> with independent <administrative censoring>. The PDF starts cohort A in 2006, not the corrupted 2000 date in the TeX aid. During the first follow-up year the pooled sample begins with 200 subjects and has $28+31=59$ deaths, with no earlier censoring. By part (a),
$$
\boxed{\widehat F(1)=\frac{200-59}{200}=0.705.}
$$
At one year, the 69 remaining cohort-B subjects are administratively censored. Cohort A supplies 72 subjects at risk for the second follow-up year, with 24 deaths and 48 survivors. Therefore
$$
\boxed{\widehat F(2)=\frac{141}{200}\frac{48}{72}=0.470.}
$$
The given grouped totals suffice because there is no censoring inside either follow-up interval; no assumption about the ordering or ties of its deaths is needed.
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