How many half lives have passed if the parent:daughter ratio is 1:3?

After one half life it will be 1/2 to 1/2 - or 1:1.





After two half lives it will be 1/4 parent to 3/4 daughter - or 1:3.





After three half lives it will be 1/8 parent to 7/8 daughter, or 1:7.