Radioactive decay expression is stated as:
N(t) = N(o) e^(-0.693t/t1/2)
N(t) = Amount remaining after time t, N(o) = Initial amount at t=0, t1/2 = Half-life.
This expression is of exponential type and thus there is an amount remaining at any given time.
At t = 120 minutes, the amount remaining can be calculated as follows:
Substituting for the values given;
N(160) = 200*e^(-0.693*120/20) = 3.13 g
