How much Cu-60 half-life 24 min would remain from a 4 mg sample after 48 min?

Answer 1

1mg. If the half life is 24 minutes then in 48 minutes the sample would decay twice. After one half life the mass would halve (to 2mg) and after the second it would have again (to 1mg).

Answer 2

After 48 minutes, two half-lives have passed for Cu-60 (24 min * 2 = 48 min). Each half-life reduces the amount of Cu-60 by half, so after 48 minutes, only 1/4 of the original 4 mg sample would remain, which is 1 mg.