the half life of the isotope is calculated as follows
the time difference = 2.00- 1.00 = 1 hour(60min)
if 20 g decay in 1 hour (60min) and 5 grams remains then the mass that decay = 20-5 = 15 grams
the half life is time required for a quantity to reduce to half its initial value
and if the initial concentration = 20g then the half = 20/2 = 10g
then time = 10g x 60min/15g = 40 min