Carbon-14 is a radioactive compound that occurs naturally in all living organisms, with the amount in the organism constantly renewed. After death, no new carbon-14 is acquired and the amount in the organism begins to decay exponentially. If the half-life of carbon-14 is 5730 yr, how old is a mummy having only 45 % of the normal amount of carbon-14?
I know this is A=AoPe^(rt) but I cant quite get it, I know the answer is 6600 years but not the method. Half life goes up as percentage goes down?