Determine the total time that must elapse until only ¼ of an original sample of the radioisotope Rn-222 remains unchanged.

Answer 1

What's the half-life of Rn222? I think that's approx 4 days but you can look that information up in your text/notes.
It takes 1 half life to reduce the amount to 1/2 of what you started and another (to make 2 half lives) to reduce another 1/2 or 1/4 (that's 1/2 x 1/2 = 1/4) of the original.