Here's what I did:
A=P(index0)10^r
80=90^r
ln80=ln90^r
r=ln80/ln90
so I get r= 0.973 but actually the solution is supposed to be r=log(8/9) or -0.051153.
Here's what I did:
A=P(index0)10^r
80=90(10^r)
ln80=ln90^r
r=ln80/ln90
so I get r= 0.973 but actually the solution is supposed to be r=log(8/9) or -0.051153.