Question

the half-life of Zn-71 is 2.4 minutes. if one had 100.0 grams at the beginning, how many grams would be left after 7.2 minutes has elapsed?

Answer 1

7.2 minutes is exactly 3 half-lives (3*2.4 = 7.2). For each half-life, we divide the amount we have by 2. 
1st half-life = 100.0/2 = 50.00 g
2nd half-life = 50.00/2 = 25.00 g
3rd half-life = 25.00/2 = 12.50 g
So we have 12.30 g after 7.2 minutes.