Question 1156567
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<pre>

The decay equation in this case is


    M = {{{M[0]*(1-0.0115)^t}}} = {{{M[0]*0.9885^t}}},


where {{{M[0]}}} is the original mass, M is the current mass, t is the time in days.


They want you find "t"  from the condition  M = {{{0.5*M[0]}}}.


It gives


    0.5 = {{{0.9885^t}}}.


Take the logarithm base 10 from both sides


    log(0.5) = t*log(0.9885).


Hence,  t = {{{log((0.5))/log((0.9885))}}} = 59.93 = 60 days  (rounded as required).    <U>ANSWER</U>
</pre>

Solved.