Jacobium is a radioactive metal with half life of 20 years. it contains 128 radioactive atoms. how many years will it take to decay to 2 radioactive atoms

N₀ = number of radioactive atoms initially present = 128

N = number of radioactive atoms left after time "t" = 2

t = time taken

T = half life period for the radioactive metal = 20 years

λ = decay constant = ?

decay constant is given as

λ = 0.693/T

inserting the values

λ = 0.693/20

λ = 0.0347

number of radioactive atoms left after time "t" is given as

N = N₀

inserting the values

2 = (128)

t = 120 years

5 w for 1hr = 5x60x60 joules=300x60=3000x6=18,000j as total energy conversion. efficiency of bulb needed for radiant to thermal within 18000