Sodium-24 has a half life how much sodium-24 will remain in an 18 g sample after 75 hours

Explanation:

The half-life of Na-24 (15 h) is the time it takes for half of it to decay.  

After one half-life, half (50 %) of the original amount will remain.  

After a second half-life, half of that amount (25 %) will remain, and so on.  

We can construct a table as follows:  

  No. of                 Fraction          Mass

half-lives   t/da   Remaining   Remaining/g

      0             0           1                      18

       1            15            ½                    9.0

      2           30            ¼                    4.5

      3           45            ⅛                    2.2

      4           60            ⅟₁₆                   1.1

      5           75            ⅟₃₂                   0.56

      6           90           ⅟₆₄                   0.28

We see that   remain after five half-lives (75 h).