Answer: Obviously the right answer is 49.6 grams
Explanation: When you know what the half life is, how much time has passed and how many grams of a sample there is left, the only thing you really need to do is figure out how many half lifes have passed during the 15.2 days. Here's how you do that:
15.2 / 3.8 = 4
So 4 half lifes have passed since then. That means the weight of the Radon-222 has decreased by 1/2 four times. In order to figure out how much we started with, we need to do this:
3.1 (our starting number!) x 2 = 6.2 grams
6.2 x 2 = 12.4 grams
12.4 x 2 = 24.8 grams
24.8 x 2 = 49.6 grams
Hope this helped!
