Intervals

If you need to delineate some section of real numbers, then you'll use an interval. For example, you want to say that the opening hours of the "Coolshop" store are from 9:00 (included) until 18:00 (included). With intervals, you would note down this fact as [9,18]. For example, the number 19\:(19:00) is not in this interval; therefore, you cannot buy anything in this shop because it's closed. The number 12.5 (i.e. half past twelve) is included in the interval, so you can shop.

A common problem are brackets, whether they are square, i.e. "[" or "]" or round, i.e. "(" or ")". But I'll tell you more about this on page 95, where I will show it to you on real-life examples.