Q: How much of a 1g sample of pure radioactive matter would be left after four half-lives?

Ans: - Given mass of sample of pure radioactive matter = 1g           It remains After one half life 1T1/2   =   1/2g=0.5g           It remains After two half life 2T1/2   =   0.25g           It remains After three half life 3T1/2 =    0.125g          It remains After four half life 4T1/2    =    0.0625g   
It would be 0.0625 g after four half lives.