For a, we have 42in/9ft. Since 12 inches are in 1 foot, we can multiply it by that to get (42in/9ft)*(1ft/12inches) = 42/108. We put the foot on the top of 1ft/12in to cancel them out. Since 2 goes into both 42 and 108, we divide it by 2/2 to get 26/54. Repeating the process again, we get 13/27
If we were to say that there are 12 inches in a foot and therefore divide 42 by 12 (to get 42 inches in feet), we'd get the same answer due to that 1 foot is equal to 12 inches and if you were to divide it by 1 foot/12 inches, since they are the same thing, you get the same answer
For c, since it may be hard to get (42/12)/9, it could just be easier to have 42/(9*12)=42/108