Deduce the formula of a simple compound from the relative numbers of atoms present in a model or a diagrammatic representation

Write and interpret general formulae of compounds in the same homologous series, limited to: (a) alkanes, (C_nH_{2n+2}) (b) alkenes, (C_nH_{2n}) (c) alcohols, (C_nH_{2n+1}OH) (d) carboxylic acids, (C_nH_{2n+1}COOH)