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For a radio active material its activity A, and the rate of change of the activity R are defined:
A=-dN/dt R=-dA/dt where N(t)=the number of nuclei at time t
Two radioactive sources P (mean life=tau) and Q (mean life=2tau) have the same activity at t=0. Their rates of change of their activity RP and RQ at t=2tau are such that RP/RQ=n/e, n=? (n can be any number between 1 and 9, inclusively.)

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