la t=t,injumatatire---> c=c,i/2
c,i=10g t,injumatatire= 8zile
in 40 zile intra 5 intervale a cate 8 zile
dupa 8zile..........c=10/2=5g
dupa in ca 8zile c=5/2=2,5g
dupa inca 8zile c= 2.5/2=1,25g
dupa inca 8 zile, c= 1,25/2=0,625g
dupa inca 8zile c= 0,625/2=0,3125g