Mike raises mink, which reproduce exponentially. He currently has 50 mink in his garage, where a month ago he had only 40.

How long does it take the number of mink to double?

ok so the rate is $\displaystyle (50-40)/40 = 0.25$

$\displaystyle 100 = 50e^{0.25T}$

$\displaystyle

T=4ln2

$ <---- is this 4ln2 month or 4ln2 year?