According to the BODMAS rule:
--->h/rt=(h/r)*t=(ht/r)
-----------------------------
ln(p)=ln(a)-(ht/r)
(ht/r)=ln(a)-ln(p)
(ht/r)=ln(a/p)
ht=r*ln(a/p)
t=[r*ln(a/p)]/h