Question 182244
At time t=0, you have "a" units of iodine since
{{{y=a*e^(-0.0856(0))=a*(1)=a}}}
Half life is the time it takes to reach 1/2 of your original amount. 
Since you started with "a" units, how long will it take to get to "a/2" units?
{{{a/2=a*e^(-0.0856t)}}}
{{{1/2=e^(-0.0856t)}}}
{{{ln(1/2)=-0.0856t}}}
{{{t=ln(1/2)/(-0.0856)}}}
or approximately
{{{t=(-0.693)/(-0.0856)}}}
{{{t=8.1}}}
It takes 8.1 time units to get to 1/2 of the original amount of iodine. 
The time unit is dependent on how 0.0856 is originally defined.