We can use two equations for this problem.
t1/2 = ln 2 / λ = 0.693 / λ
Where t1/2 is the half-life of the element and λ is decay constant.
20 days = 0.693 / λ
λ = 0.693 / 20 days (1)
Nt = Nο eΛ(-λt) (2)
Where Nt is atoms at t time, No is the initial amount of substance, λ is decay constant and t is the time taken.
t = 40 days
No = 200 g
From (1) and (2),
Nt = 200 g eΛ(-(0.693 / 20 days) 40 days)
Nt = 50.01 g
Hence, 50.01 grams of isotope will remain after 40 days.