Answer :

mreijepatmat
B= P.e^(9rt)
ln(B) =ln(P.e^(9rt)
ln(B) = ln(P)+lne^(9rt)
lnB() = ln(P) + 9rt.lne
ln(B) = lnP() + 9rt
ln(B) - ln(P) = 9rt and t = [ln(B)-ln(P)]/9r or t= [ln(B/P)]/9r (both are correct)