For a radioactive material, its activity A and rate of change of is activity R are defined as $A=-\frac{dN}{dt}$ and $R=\frac{dA}{dt}$ , where N(t) is the number of nuclei at t . Two radioactive source P (mean life T) and Q (mean life 2T) have the same activity at t = 0. Their rate of change of activities at t = 2T are R_P and R_Q respectively. If $\frac{R_P}{R_Q}=\frac{n}{e}$, then the value of n is