The general way to work this out is to solve the general expression for the remaining quantity versus half-life, using logarithms. But that's not necessary with these numbers.
Look at the numbers:
-- 3 mg is 1/4 of 12 mg.
-- 1/4 is the product of (1/2) x (1/2).
-- So the 3 mg is what's left of 12 mg after 2 half-lives. The 26 minutes must be two half-lives.
-- The half-life of that substance is 26/2 = <em>13 minutes</em>.
ar some point pattern A will have the same numbers as pattern B as it moves along like if you add 3 for 2 more times in pattern A it will have 24 and so on..
