What Was the Angle Between Two Objects in an Inelastic Collision?

[AERam]

This problem's got me stumped.

"After a completely inelastic collision between two objects of equal mass, each having initial speed v, the two move off together with speed v/3. What was the angle between their initial directions?"

Any help would be appreciated!

 

[Tide]

Use momentum conservation. You should find that

\cos \frac {\theta}{2} = \frac {1}{3}