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 __________.