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)
