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.