Solution
= Solution
The <Nelson–Aalen estimator> is
$$
\widehat H(t)=\sum_{t_j\leq t}\frac{d_j}{r_j}.
$$
For tied events, $d_j/r_j$ uses the number of events sharing time $t_j$ and the risk-set size just before that time.
At month 5,
$$
\widehat H_A(5)=\frac15+\frac14+1=\frac{29}{20},
\qquad
\widehat H_B(5)=\frac24=\frac12.
$$
Their ratio is $29/10=2.9$.