Relativistic mass question

[tardis]

Hi all, would be very grateful if someone could give me some pointers on this.

If you have two particles of equal rest mass travelling in the same direction, particle A travelling at say 0.5c and particle B behind it travelling at 0.6c and they eventually collide inelastically, how do you calculate the mass of the 'combined particle', as seen by a stationary observer?

Thanks.