Please look at pictures. There is something really weird going on. They say the bob has to make a vertical circle. Then, in the solutions, they say that, for that to happen, speed at top must be greater than zero. BUT, since the only force acting is mg, that means that mg=mv^2/r. <=> v=sqroot(rg) This is my solution: vbob+bullet=sqroot(rg)=4.43m/s. m1 vbullet=(m1+m2) vbob+bullet. vbullet=137 m/s. Who is right?