Radium-223 has a half-life of 11.4 days. Approximately how long would it take for the activity of a sample of 223Ra to decrease to 2.00 % of its initial value?

Answer 1

Refer to your other posts.
Almost all of these can be worked with two thoughts.
#1.
k = 0.693/t_{1/2
#2. ln(No/N) = kt where No = initial and N = final, k is from the #1 calculation and t1/2 is the half life.}