IMO 1990 Problem 4

Let ${\mathbb Q}^ +$ be the set of positive rational numbers. Construct a function $f : {\mathbb Q}^ + \rightarrow {\mathbb Q}^ +$ such that $$f(xf(y)) = \frac {f(x)}{y}$$ for all $x$ , $y$ in ${\mathbb Q}^ +$ .