  In the nonextensive Tsallis scenario, Page's conjecture for the average
entropy of a subsystem[Phys. Rev. Lett. {\bf 71}, 1291(1993)] as well as its
demonstration are generalized, i.e., when a pure quantum system, whose Hilbert
space dimension is $mn$, is considered, the average Tsallis entropy of an
$m$-dimensional subsystem is obtained. This demonstration is expected to be
useful to study systems where the usual entropy does not give satisfactory
results.
