Atoms and Nuclei

In a radioactive decay process, the activity is defined as A=−‌

dN

dt

, where N(t) is the number of radioactive nuclei at time (t). Two radioactive sources, S1 and S2 have same activity at time t=0. At a later time, the activities of S1 and S2 are A1 and A2, respectively. When S1 and S2 have just completed their 3‌rd ‌ and 7‌th ‌ half-lives, respectively, the ratio ‌

A1

A2

is ______________.