1) If the half-life of ¹⁴⁰₅₆Ba is 12.75 days, how many atoms of  ¹⁴⁰₅₆Ba will be left after
    9 weeks if there were initially 2.4 mol of atoms?

    4.94 halflives   //       4.7 x 10²² atoms
                 

2) How long will it take 3.8 x 10²⁴ atoms of ¹⁰⁰₄₇Ag to disintegrate to 2.93 x 10²³ atoms
    if the half life of  ¹⁰⁰₄₇Ag is 24.6 seconds?

     2^#halflives = 12.97    //   3.7 halflives    //   90.9 sec
  

3) Radioactive copper, ⁶⁴₂₉Cu, is found in quantities exceeding pollution standards in the 
    sediments of a reservoir in a routine check on Monday.  The standard allows up to
    14 ppm/cubic meter of sediment.  On Monday, 59ppm/cubic meter were measured.
    The half life of ⁶⁴₂₉Cu is 12.7 hours.  When will the pollution level return to 14ppm?

    2^#halflives = 4.2   // 2.07 halflives   //  26.3 hours later, or Tuesday at about the same time
   

4) An unidentified corpse was discovered on 21 April at 7:00 AM.  The pathologist
    discovered that there were 1.24 x 10³⁷ atoms of ³²₁₅P remaining in the victim's bones
    and placed time of death sometime on 15 March.  The half life of ³²₁₅P is 14.28 days.
    How much ³²₁₅P was present in the bones at the time of death?

    15 March --> 21 April is 38 days   

    2.66 halflives   //   7.84 x 10³⁷ atoms