The half-life of phosphorus-32 is 14.30 days. how many milligrams of a 20.00 mg sample of phosphorus-32 will remain after 85.80 days?

The amount of sample that is left after a certain period of time, given the half-life, h, can be calculated through the equation.

             A(t) = A(o) (1/2)^(t/d)

where t is the certain period of time. Substituting the known values,

             A(t) = (20 mg)(1/2)^(85.80/14.30)

Solving,

           A(t) = 0.3125 mg

Hence, the answer is 0.3125 mg.