Cobalt-60, with a half-life of 5.0 days, is used in cancer radiation treatments. A hospital purchases a 30.0g supply of it and stores it on a shelf in a closet. The next time a hospital worker grabs the bottle, though, they find that it only contains 3.75 g remaining! Assuming the bottle had never been opened, how long was the bottle sitting on the shelf before the worked grabbed it?