From which water source would you drink from? Why?

The  number of  years  required  for 1/4  cobalt-60  to remain   after   decay  is calculated  as follows

  after  one  half life  1/2 of the original  mass isotope  remains

after  another half life 1/4  mass  of  original  mass  remains

therefore  if one  half  life is  5.3   years  then  the  years required

= 2  x 5.3years =  10.6   years