Two radioactive materials A and B have decay constants `5lambda` and `lambda` respectively. Initially both A and B have the same number of nuclei. The ratio of the number of nuclei of A to that of B will be `1/e`, after the time `x/(8lambda)` then x is :

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.