Which object would have the most momentum?
a. a 500-pound car moving at 20 miles per hour
b. a 200-pound car moving at 60 miles per hour
c. a 100-pound car moving at 80 miles per hour
d. a 50-pound car moving at 100 miles per hour

Momentum is directly proportional to mass and speed.

When working with SI units, the interoperability of the units
is all taken care of.  The proportionality constant is ' 1 ', and
we can simply say   
                                Momentum = (mass, in kg) x (speed, in m/s) .

In other systems of units, such as the ponderous and complex
'customary' one referenced in this question, the proportionality
constant isn't ' 1 ', but momentum is still directly proportional
to mass and speed.  So if all we need to find is the greatest
momentum from a list, we can simply look for the greatest
product and not worry about the units.  The actual numbers
won't mean much, but each product is still directly proportional
to the mass and speed that are its factors, so the greatest
momentum can easily be identified.

a).  500 x 20  =  10,000 'pound-mph'
b).  200 x 60  =  12,000
c).  100 x 80  =    8,000
d).  50 x 100  =    5,000

(b). has the greatest product, and therefore the greatest momentum.
For a meaningful number, we'd still need to do some work on the units.