Two radioactive materials X1 and X2 have decay constants 10 λ and λ respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of X1 to that of X2 will be 1/e after a time 1xλ. The value of x is
Nx1(t)Nx2(t)=1e or N0e−10λtN0e−λt=1e
(Initially both have same number of nuclei say N0)
or e=e−λte−10λt
or e=e9λt
or 9λt=1
or t=19λ