11 feet 8 inches plus 2 feet 9 inches. This confuses me.

The answer wold be 13ft and 17inch

So their is 12 inches in 1 foot which you should know. If you do the simple part first, which is to add 11 feet plus 2 feet then you get 13 feet. You then add 8 inches and 9 inches which is 17 inches but that's over 12 so you subtract 12 from 17 and you are left with 5 but that subtracted 12 gets added to the 13 feet by adding 1 single foot. The final answer should be 14 feet 5 inches.