use the nuclear decay equation to find out how much a 7mg sample of iodine 128 remains after 116 minutes if the half life of this isotope is 25 minutes.
k = 0.693/t1/2
k = 0.693/25 min = ??
Substitute k into the equation below.
ln(No/N) = kt
No = 7 mg
N = unknown
k = from above.
t = 116 minutes.
Solve for N.