a 50-mg sample of cobalt-60 decays to 40mg after 1.6min. determine the half-life of the cobalt-60.

this is as far as i got before getting stuck

M = C

40 = 50

0.8 =

=

then right here i get stuck and can't get the correct answer. can somone tell me what i'm doing wrong?

i'm getting the answer 0.52 mins, when its susposed to be 5 mins