Exponential and log models
An exponential model multiplies by a fixed factor each period, so take logs when the unknown sits in the exponent.
A worked example
A 400 mg sample with a 6-hour half-life has passed three half-lives after 18 hours, leaving 400 x (1/2)^3 = 50 mg.
The mistake people actually make
Count how many half-lives have gone by rather than halving once. log A - log B is log(A/B), not log A divided by log B.