A radioactive 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