The activity of a sample of a radioactive material is A1, at time t1 and A2 at time t2 (t2>t1). If its mean life T, then 
(a) A1t1=A2t2            (b) A1-A2=t2-t1
(c) A2=A1et1-t2/T    (d) A2=A1et1/t2T

(c) A=A0e-λt=A0e-t/τ ;  where τ = mean life

So A1=A0e-t1/TA0=A1e-t1/T=A1et1/T

A2=A0e-t/T=(A1et1/T)e-t2/TA2=A1e(t1-t2)/T

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