Let mass and velocity of van and motorcycle be m1, v1 and m2, v2 respectively. Since they have same momentum, (mxv)1 = (mxv)2. v1 = (m x v)2/m1 . Since m1 >> m2, velocity of van (v1) should be less than velocity of motorcycle (v2) for momentum to be same. Hope you got what I mean.
