If C-11 has a half life of 20 minutes and you have 2 grams what will be left in an hour?

0.0015 If I am mistaken, this needs to be edited out. Half life of one hour means that half the material will be left after one hour. Over the next hour half of that amount is left, etc. So over three hours, you would have 1/2 X 1/2 X 1/2, or one eighth of the material left. 1/8 is .125 of the original gram.