Gravitational Force: Find Angle for Net F on Particle 1

[divr]

Fig1 shows an arrangement of five particles with masses M1 = 10.0 Kg, M2=M3=M4=M5=2.5 Kg and with r = 4 cm. For what angle (in degrees) of the net gravitational force F on particle 1 due to other particles cancels out.
a)30 b) 45 c) 90 d) 0

(figure is given in the attachment)

 

[Doc Al]

Well, what do you think? Hint: Can you simplify the diagram at all?