Maggie was doing research for her ph. d. in physics when a 12 mg sample of radioactive material decayed to 3 mg in 26 minutes. determine the half life of this substance.

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 = 13 minutes.

Go Maggie !