If radon-222 has a half-life of 3.8 days, determine how long it takes for only 1/8 of a sample to remain.

Answer 1

Find k from the following.
k = 0.693/t_1/2

Then substitute into the following
ln(No/N) = kt
Use a convenient number for No, then N will be 1/8 of that, solve for t.