By how many atomic mass units do subsequent members of homologous series differ?

Subsequent members of a homologous series differ by 14 atomic mass units (amu). This is because each member of a homologous series differs from the previous one by a methylene group (CH2). The atomic mass of a carbon atom (C) is 12 amu, and the atomic mass of two hydrogen atoms (H2) is approximately 2 amu (1 amu each). So, a CH2 group adds a total of 12 amu (C) + 2 amu (H2) = 14 amu to the molecular mass of the previous member in the homologous series.