Set up the problem so you can visualize the equation you must perform. For example, if you wish to add 3 feet 8 inches, 8 yards and 4 feet 6 inches, write the measurements in a vertical problem aligning the like units. Similarly, if you wish to subtract 3 feet 5 inches from 9 feet 7 inches, set up the problem vertically and align the like units.
Work from right to left to add or subtract. Add or subtract the smallest unit first. In the addition problem, add 8 inches and 6 inches together to get 14 inches. Add 3 feet and 4 feet together to get 7 feet. You have nothing to add to the 8 yards. In the subtraction problem, subtract 5 inches from 7 inches to get 2 inches. Subtract 3 feet from 9 feet to get 6 feet.
Check your answers to see if you can rename the units. In the addition problem, you have 14 inches, 7 feet and 8 yards. Rename 14 inches to make an additional foot with 2 inches left over. This makes the final answer to this addition problem 8 yards, 8 feet and 2 inches. In the subtraction problem, the final answer is 6 feet, 2 inches.
Check your answer against a conversion chart, if necessary, to ensure that you rename your answer into the correct units. For example, if you are adding 4,035 feet and 1,500 feet together, the answer is 5,535 feet. Because 1 mile equals 5,280 feet, subtract 5,280 from 5,535 to get 255. You can rename this answer to 1 mile, 255 feet.