You have a 10 foot piece of pipe that you are dividing into smaller pieces so division is the proper operation. Each of the pieces is 9/5 feet long which is a little shorter that 2 feet. If the pieces were 2 feet long then you would get exactly 5 pieces since 10/2 = 5. But the pieces are not 2 feet long they are 9/2 feet long. Thus you need to divide 10 feet by 9/2 feet.
When you do this division you will get a number larger than 5, but is it larger than 6? Do you get 6 pieces or only 5?
There is another way to look at this problem that involves multiplication. Again since 9/5 feet is a little shorter that 2 feet I know that I will get at least 5 pieces. How much of the 10 feet is used up by 5 pieces, each 9/5 feet long? 5 9/5 = 9 so 5 pieces will use up 9 feet of pipe. This only leaves one foot, which is not enough to make another piece.