  Given a simplicial pair $(X,A)$, a simplicial complex $Y$, and a map $f:A \to
Y$, does $f$ have an extension to $X$? We show that for a fixed $Y$, this
question is algorithmically decidable for all $X$, $A$, and $f$ if $Y$ has the
rational homotopy type of an H-space. As a corollary, many questions related to
bundle structures over a finite complex are likely decidable. Conversely, for
all other $Y$, the question is at least as hard as certain special cases of
Hilbert's tenth problem which are known or suspected to be undecidable.
