There will be 1.11 grams of the sample of radon-222 left. This answer can be obtained using the formula of half life to get the rate constant which will be used in another equation later on.
Half-life (t) = ln2/k = 3.8 days
k = 0.182407/day
Using the general equation of a first order reaction:
Ca/Cao = 1/e^(kt)
Ca/Cao = 0.01506 --> fraction of radon-222 left
This means that 1.51% of the original amount remains, so 1.51% of 73.9 is 1.11 grams.
