the half life of F18 is 2.089x10^-4yrs. How long would it take (in days) for F18 to decrease to 78.71% of its initial amount?
k = 0.693/t1/2. Substitute k into the following:
ln(No/N) = kt
I would choose a convenient number like 100 for No, then N will be 78.71. Substitute these and k into the ln equation and solve for t. The units will be years and you must change that to days. You know there are 365 days in a year.