SOLUTION: a car is going 35 mph, how fast is it going in cm per second?

 
There are 5280 ft in a mile so we make the unit fraction 
putting the miles on the bottom to cancel away the miles in the top, and
we multiply by that unit fraction:
 

 
and we cancel the miles:
 

 

There are 60 minutes in an hour so we make the unit fraction 
putting the hours on the top to cancel away the hours on the bottom, and
we multiply by that unit fraction:
 

 

and we cancel the hours:
 

 
There are 12 inches in a foot so we make the unit fraction 
putting the feet on the bottom to cancel away the feet on the top, and
we multiply by that unit fraction:
 

 

and we cancel the feet:
 

 
There are 60 seconds in a minute so we make the unit fraction  
putting the minutes on the top to cancel away the minutes on the
bottom, and we multiply by that unit fraction:
 

 
and we cancel the minutes:
 

 

There are 2.54 centimeters in an inch so we make the unit fraction 
putting the inches on the bottom to cancel away the inches on the 
top, and we multiply by that unit fraction:
 

 
and we cancel the inches:
 






Edwin