27. (Contributed by John Oprea of Cleveland State University.) The Nuclear Test-Ban Treaty of 1963 brought an end to atmospheric testing of nuclear weapons (except by France and China). Testing had released into the atmosphere the radioactive isotope strontium-90. Just as with iodine-131, it settled out of the air onto grass in fields, was eaten by cows, and wound up in children’s milk. In the body, strontium-90 mimics calcium and is absorbed into the bones, where its radiation can cause cancer; its half-life is 28.8 years. Assuming that none of the strontium-90 absorbed into the bones of children in 1962 has been otherwise excreted, approximately what proportion will still remain 58 years later, in 2020?