Question 981571
a=ao e ^(-4.132 t)

a=(1/2) ao
Therefore 
(1/2)=e^(-4.132 t)
ln of both sides
-0.693= -4.132 t
divide by (-4.132)
t=0.1677 days

check
-4.132*0.1677=-0.6929
e^-0.6929=0.5