Advanced Physics

2 particles of mass 0.2kg and 0.3kg are placed 0.15 meters apart. A third particle of mass 0.05kg is placed between them on the line joining the first two particles. Calculate:
A) The gravitational force acting on the third particle if it were placed 0.05 m from the 0.3...... I answered that one already the real problem that I am having is with question B.
B) Where along the line should it be placed for no gravitational force to be exerted on it. -----would really appreciate some help with this question