TP 32.25

Given: T_1/2 = 122 s

N/N₀ = 10^-4

Find: t - ?

l = ln (2) / T_1/2

l = ln (2) / (122 s) = 0.00568 s^-1

N = N₀ e ^{–l t}

 N/N₀ = e ^{–l t}

ln (N/N₀) = ln (e ^{–l t})

ln (N/N₀) = –l t

t = ln (N/N₀)/(–l)

t = ln (10^-4)/(– 0.00568 s^-1) = 1621 s