Half life can be used to solve for the amount (A) of a substance left after t time by using the following equation:

A=A^0⋅(12)^t/h
If the half life of a substance is 4 weeks, how long will it take for 400 mg of this substance to decay to 25 mg?

Select one:
A. 8 weeks
B. 13 weeks
C. 16 weeks
D. 6 weeks