A sample of radioactive isotopes contains two different nuclides, labeled A and B. Initially, the sample contains the same number of nuclei A as B (i.e. the sample composition is A/B = 1). The half-life of A is 3 hours and that of B is 6 hours. What is the expected composition of the sample after 18 hours? That is, what would be the value of the ratio A/B between the number of nuclei of type A and B?