A population doubles every 15 years. Determine the yearly growth rate

A radioactive substance decays from 400 grams to 120 grams in 320 years. Determine it's half life

Hey guys, as far as i know the above problems should be solvable with the formula A=I*r^(t/p)

A being the final value

I being the initial value

r being the rate of growth/decay

t being the time

and p being the growth/decay period per rate

It would be great if anyone can help. Thanks in advance