In a radioactive decay process, the activity is defined as  $A=-\frac{dN}{dt}$, where N(t) is the number of radioactive nuclei at time t. Two radioactive sources,  $S_1$  and  $S_2$  have same activity at time t= 0. At a later time, the activities of   $S_1$  and $S_2$  are  $A_1$ and  $A_2$  respectively. When $S_1$  and  $S_2$   have just completed their 4th and 7th half-lives, respectively, the ratio  $A_1/A_2$  is ___________.