Aradioactive sample is 555 years old. it’s half life is 111 years. how many half lives?

We are going to use the half-life formula:

the no. of half-lives =  the total time/time for one half-life

when the total time = 555 years

and the time for one half-life = 111 years

so by substitution:

∴ no. of half-lives = 555 / 111 

                              = 5