[SPACE] or click green flag to enter number. These are simple implementations of rounding. "Simple" in this case is being easy to build with only a few of Scratch's blocks, exploiting the behavior of numbers. Numbers in almost every device follow the same standard, which also assumes behavior that can be exploited to create efficient rounding methods. The details are very long to go over, so I'll leave the studying over to you. Some of these implementations can also be found in other languages such as C# (from what I know) and are used in professional development contexts.
