Rounding
An exact number often carries more detail than the moment needs. Which simpler, nearby number should stand in for it?
Where you are 3 inside
What this area is about
Rounding replaces a number with the closest number that has only zeros from a chosen place onward. The number sits between two such neighbors, and a halfway mark between them decides which is closer; a number exactly on the halfway mark rounds up. The idea is the same for whole numbers and for decimals.
How these fit together
All three are procedures, each widening the one before.
Rounding to the nearest ten or hundred finds the two neighbors on a number line and compares distances.
Rounding to any place carries that to thousands and beyond, including roundings that carry into the next place, such as 396{,}500 \approx 400{,}000 to the nearest ten thousand.
Rounding decimals rounds to the nearest whole number, tenth or hundredth, as when 7.648 \approx 7.6 to the nearest tenth.
A path through it
Tens and hundreds first, then any place, then decimals, which also need decimal place value.
Finish here and you can estimate sums and differences, estimate by rounding before multiplying, and see why a measurement is only as exact as the tool that made it.