SOLUTION: Calvin weighs 70 pounds and is walking in the front door at a rate of 3 ft/s. He is tackled by Hobbes running in the opposite direction at 15 ft/s. If Hobbes weighs 120 lbs, what i

Since the problem asks about the resultant speed of both, it means that after collision, they move 
as one single body with the common speed.


    In Physics, such collision is called "inelastic" collision.


For such collisions, the Linear Moment conservation low is applicable:


     -  = .    (1)


In formula (1),  and  are their masses;   and  are their velocities before the collision;

the sum   is the total mass of the "glued" body and  is their common speed after collision.


The sign " - " in formula (1) says that the bodies move initially in opposite directions.

The sign of the sum in the left side of (1) determine the direction, in which the resulting mass does move.


Substitute the given data into the formula (1)

    70*3 - 120*5 = (70+120)*V


and get

    V =  = -2.053.


ANSWER.  The resulting speed is -2.053 ft/s.  They move together in the same direction, as Hobbes moved before the collision.