Question 978662
A (t) = A0e^-0.087t
A(t)/Ao= 0.25, because A(t) is the new level, and that is 25% of the original level.

0.25=e^(-0.087t)

take ln of both sides
-1.38629= -0.087t

t=15.93 days

e^(-0.087*16)=0.2485.  This makes sense.