Radon-222 3.8 days How many half lives have occurred if 12.5% of Rn-222 remains?

I need someone to explain how you do this to me?

3 half-lives have occured

Given

Half-life Radon-222 = 3.8 days

12.5% of Rn-222 remains

Required

Half-lives  occurred

Solution

General formulas used in decay:  

T = duration of decay  

t 1/2 = half-life  

N₀ = the number of initial radioactive atoms  

Nt = the number of radioactive atoms left after decaying during T time  

12.5% of Rn-222 remains ⇒ Nt/No = 0.125

Input the value :

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)