Use the fact that the quantity of a radioactive substance after t years is given by q = q0(2−t/k),
where q0 is the original amount of radioactive material and k is its half-life (the number of years it takes for half the radioactive substance to decay).

The half-life of strontium-90 is 25 years. Find the amount of strontium-90 remaining after 20 years if
q0 = 87 kg.
(Round your answer to the nearest kg.)