Two radioactive materials A and B have decay constants `10lambda and lambda`, respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of A of that of B will be `1//e` after a time :

Explanation

Using the law of radioactive decay, one can write `(N_(A)(t))/(N_(B)(t))=(N_(0)exp(-5lambdat))/(N_(0)exp(-lambdat))=1/e`. Solving this one gets the result.