Half life of a certain radioactive material is 100 days. After how much time the undecayed fraction of material will be 6.25%?

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% ?

Half-life of a certain radioactive materal against alpha -decay is 138 days. After what lapse of time the undecayed fraction of the material will be 6.25 % ?

Half life of a radioactive sample is 4 days. After 16 days how much quantity of matter remain undecayed :