To solve this, we can use two equations.
t1/2 = ln 2 / λ = 0.693 / λ - (1)
where, t1/2 is half-life and λ is the decay constant.
Nt = Nοe∧(-λt) - (2)
Nt = amount of atoms at t =t time
Nο = initial amount of atom
t = time taken
From (1),
10 yr = 0.693 / λ
λ = 0.693 / 10 yr
From (2)
Nt = 100 g eΛ(-(0.693 / 10 yr) x 20 yr)
Nt = 25.00 g.
Hence, remaining mass will be 25.00 g
