Two uniform solid spheres of equal radii R, but mass M and 4 M have a centre to centre separation 6 R as shown in the figure.

The two spheres are held fixed. A projectile of mass ' m' is projected with minimum velocity from the surface of the sphere of mass M directly towards the centre of the second sphere so that it reaches the surface of the second sphere. If the speed with which the projectile reaches the surface of the sphere of mass 4M is $\sqrt{\frac{xGM}{5R}}$ find x.