Question

if a 84g sample of thorium decays with a 10 minute half life, how much would the sample weight in a half hour?

Answer 1

half hour = 30 min = 3 half lives
84 * 1/2 * 1/2 * 1/2 = 84/8 = 10.5g

or in general
mass = original mass /2^n
where n is the number of half lives