I'm reviewing this question and think you can't find the answer with the information given.
A patient receives 1g of drug X as a single oral dose. Drug X has a half-life of 12 hours and an oral bioavailability of 40%. What mass (in milligrams) of drug X will still be present in the body 24 hours after a 1g dose?
I think the fact that it is an oral dose means the question is more complicated, involving absorption and distribution of the drug, meaning more factors are needed which are provided in the question.
Is it as easy as: $1000mg \times 0.4 \times 0.5 \times 0.5 = 100mg$
Or there is is more it than that as I suspect? Many thanks!