if 0.78% of a radioactive substance remains, then how many half-lives have passed?

let us calculate it after the following half life the following amount of substance will remain

1.    50% will remain after half life 1^st

2.    25% will remain after half life 2^nd 

3.    12.5% will remain after half life 3^rd

4.    6.25% will remain after the 4^th half life

5.    3.125% after the 5^th half life

6.    1.5625% will remain after the 6^th  half life

7.    0.78125%  will remain after the 7^th  half life

after the 7^th half lives