To obtain final image at infinity, object which is the image formed by objective should be at focal distance of eyepiece. By lens formula (for objective)
1
v0
−
1
u0
=
1
f0
or,
1
v0
−
1
−25
=
1
20
⇒
1
v0
=
1
20
−
1
25
=
5−4
100
=
1
100
mm ∴v0=100mm Therefore the distance between the lenses =v0+fe=100mm+20mm=120mm