In this problem, the both initial velocities are co-directed ( both are positive).


So you calculate the total moment p = ,


and then divide it by the total mass    of the glued objects.


So, the final speed of the two GLUED objects is    .     (1)



When the units in the problem are from different systems (as kg and mph), it reminds me a ZOOPARK, not a Physics problem.


Good style requires to CONVERT and to use all the values in consistent system.


But in this case, due to the linear nature of the basic formula (1), 

you can use kg and mph (but remembering that it is not a good style !)