/sci/ - Science & Math

"A radioactive substance decays exponentially over time. If we start with a 250 gram sample there will only be 20 grams 300 years later. Create a function to model the amount of substance that remains in terms of time, measured in years. Show all work."

Y=Pe^rt

P->250 Grams
e->e
r->?
t->300 years
Y->20

20=250e^r*300
2/25=e^r*300
ln(2/25)=r*300
(Ln(2/25)/300)=r
r≈-0.008419≈0.842%

and this is the part I get tripped up on.
is this the answer? Is asking for more?

f(t)=250e^-0.00842(t)