Authors’ abstract: “Consider an equation \(y=f(x_ 1,...,x_ n)\) where the extension principle is used to find \(\bar Y=f(\bar X_ 1,...,\bar X_ n)\) when we substitute fuzzy numbers \(\bar X_ j\) for the \(x_ j\), \(1\leq j\leq n\). If \(\bar X_ j(\alpha)\) denotes an \(\alpha\)-cut of \(\bar X_ j\), then we argue that, for only certain simple equations, one will obtain \(\bar Y(\alpha\)) equal to \(f(\bar X_ 1(\alpha),..., \bar X_ n(\alpha))\) where f is evaluated using interval arithmetic.”