64 km
solution::
Escape velocity =\(\sqrt{2GM\over R}\)
we know mass remain unchanged
\(V_e' \over V_e\) =\(\sqrt{R_e\over R_e'}\)
\({{10V\over e}\over {V\over e}} ={\sqrt{6400\over R_e'}}\)
\(R_e'=64 km\)