If I work then I earn money, but if I'm idle, then I enjoy myself. Either I work or I'm idle. However, if I work, then I do not enjoy myself, while if I'm idle, then I do not earn money. Therefore,
(a) I work if and only if I enjoy myself
(b) I work if I earn money
(c) I do not earn money if I'm idle
(d) I enjoy myself if and only if I do not earn money