If 20.g of a radioactive isotope are present at 1: 00 pm and 5.0g remain at 2: 00 pm, what is the half life of the isotope?

The  half  life  of   the  isotope  is  calculated  as  follows

    the  time  difference  =  2.00- 1.00 =  1 hour(60min)

if  20  g  decay  in  1 hour (60min)  and  5  grams  remains  then the  mass  that  decay  =  20-5 = 15   grams

  the  half  life  is  time  required  for  a quantity  to  reduce  to  half  its  initial  value

and  if the  initial   concentration  = 20g then  the   half  =  20/2  =  10g

then  time  =    10g   x  60min/15g  = 40 min

Most smaller stars don't have a radiative zone.

The correct answer is "atomic weight".