How to subtract feet and inches

If the number of inches being subtracted is greater than the number of inches in the length that is being subtracted from, decrease the number of feet by one and increase the number of inches by 12 in the length from which you are subtracting.
Subtract the inches.
Subtract the feet.

Example: Subtract 3 feet 10 inches from 6 feet 8 inches.

The number of inches being subtracted is larger so adjust the number of inches:
6 feet 8 inches = 5 feet 20 inches
Subtract the inches:
20 - 10 = 10 inches
Subtract the feet:
5 - 3 = 2 feet
Answer: 2 feet 10 inches