Let the count rate at zero time be N
N 3 min→\frac{3\text{ min}}{\rightarrow}→3 min N2\frac{N}{2}2N 3 min→\frac{3\text{ min}}{\rightarrow}→3 min N4\frac{N}{4}4N
N4\frac{N}{4}4N
= 400
N = 400N4\frac{400}{\frac{N}{4}}4N400 x N
= 1600
