duono

Duono is du plus the fraction suffix -on-, and that suffix is the whole point: once you have it, triono, kvarono and centono cost you nothing. What follows a fraction takes de, so duono de la pano. As a prefix it glues straight on, giving duonhoro for half an hour and duonvoĉe for under one's breath, and it shades into "half-hearted" in words like duonfermita. Note that duonpatro is a stepfather rather than half a man, which is the sort of thing worth knowing before you use it.