Hi, Not sure if I misunderstood the lesson, but if we apply to interval rules such that: Include a number n when it is greater than the lower bound and Include a number n when it is lower or equal to the upper bound. Then it would follow that 1 would not be included in the interval 1 to 21, correct? Because 1 is not greater than the lower bound. Unless there's an exception made for the starting and ending intervals? Thanks!

Sheldon, I had this exact same question, but the assignment was able to make this clearer. If you view the solution, you will find that the first number is always included in the first interval, and the last number is also always included in the last interval. I hope this helps to clear things up!