4 half lives have occurred. Divide 20 ( the number of years it sat out ) by 5 (it’s half life). To find the remaining amount, divide the 300g by 2 four times, since that’s the amount of half lives that occurred:
300/2 = 150 (1 half life)
150/2 = 75 (2 half lives)
75/2 = 37.5 (3 half lives)
37.5/2 = 18.75 (4 half lives)
There is 18.75 g of Cobalt-60 remaining.
