Answer:
about 5.84 mg
Explanation:
The amount remaining after t days can be written as ...
remaining = initialAmount · (1/2)^(t/(halfLife))
Filling in the given numbers, you have
remaining = (40 mg)·(1/2)^(180/64.84) ≈ 5.84 mg
About 5.84 mg would be left after 180 days.