Solution
= Solution
Put $n(a,t)=A(a)T(t)$. Separation gives
$$
\frac{T'}T=-\frac{A'}A-d(a)=r
$$
for a constant $r$. Hence
$$
\boxed{T(t)=T_0e^{rt},\qquad
A(a)=A_0\exp\left(-ra-\int_0^ad(s)ds\right)}.
$$
= Solution
Put $n(a,t)=A(a)T(t)$. Separation gives
$$
\frac{T'}T=-\frac{A'}A-d(a)=r
$$
for a constant $r$. Hence
$$
\boxed{T(t)=T_0e^{rt},\qquad
A(a)=A_0\exp\left(-ra-\int_0^ad(s)ds\right)}.
$$