radioactive isotope has leaked into a small stream.Four hundred days after the​ leak, 11​% of the original amount of the substance remained. Determine the​ half-life of this radioactive isotop

Answer 1

T = half-life in days

t = days elapsed

Set this up as

0.11 = (0.5)^t/T

0.11 = (0.5)^400/T

Now solve for T and finish from here.