An object traveled 691.12 feet in 21.2 seconds.find both unit rates.  a.32.6 s/ft and approximately 0.03 ft/sb.32.6 ft/s and approximately 0.03 s/ftc.32.6 s/ft and approximately 0.03 s/ftd.32.6 ft/s and approximately 0.03 ft/s

When we divide the number of feet (~700) by the number of seconds (~20), we get a number that is more than 30 (ft/s). Thus, only choices B and D are viable.

When we invert that rate, we get 1/(30 ft/s) = (1/30) s/ft, leaving the only viable choice as ...
  B. 32.6 ft/s and approximately 0.03 s/ft


Units (feet, seconds) multiply and divide like any other variable. It is often useful to keep the units with the numbers of a problem so you can tell if you've done the math properly. It is more likely to be correct if the units are correct.