Option (b) is correct.
Explanation:
we know that
[tex] N= N_o (\frac{1}{2})^{n} [/tex]
No= initial mass of substance
N= present mass of substance
n= number of half life times
n= total time/half life time
n=20/10
n=2
so [tex] N= 100 (\frac{1}{2})^2 [/tex]
N= 100/4
N=25 grams
