After three half lives, the percentage of fraction of amount

Two radioactive substances X and Y originally have N_(1) and N_(2) nuclei respectively. Half life of X is half of the half life of Y. After three half lives of Y, number of nuclei of both are equal. (N_(1))/(N_(2)) The ratio N will be equal to :

After a time equal to four half lives, the amount of radioactive material remaining undecayed is-

After a time equal to four half lives, the amount of radioactive material remaining undecayed is-

Half life of a certain radioactive material is 130 days. After what lapse of time, the undecayed fraction of the material will be 25% ?

Half life of a certain radioactive material is 140 days. After what lapse of time, the undecayed fraction of the material will be 25% ?