Your teacher might want you to do it as a ratio and proportion
problem:

32 mph means the ratio 32 miles to 1 hour

32 miles is to 1 hour as 128 is to  ?  hours.

Put an x where the " ? " is:

32 miles is to 1 hour as 128 is to x hours.

"is to" means "over" and "as" means "equals".

32 miles over 1 hour equals 128 miles over x hours.



It's not necessary to write in the units:



Cross-multiply:

32x = 128·1

32x = 128

Divide bth sides by 32.

  x = 

  x = 4

So x hours is 4 hours.

Edwin