The half life of a isotope is 30 seconds, and you have 10 grams to start with, after one minuet, you will have;
C. 2.5 Grams
t0 = 10g
t30 = 5g Half-life 1
t60 = 2.5g Half-life 2
Half-life means that as every half-life period passes, you have half of the isotope left, compared to what was there from the beginning.
