Rounding is where you make a number have less accuracy. Examples.
1.03 = 1.1
2.59 = 2.6
2.9 = 3
2.1 = 2
We are going from 2 or 3 digit numbers (called significant figures) to a smaller number in these examples. Rounding is important for many reasons. It gives a more true value about the measurement. If I used a regular school ruler and told you that the number was 1.23134 inches, it would be unreasonable that I could get that much detail with it, so I round to 3 significant figures, which gives 1.23, to show the true accuracy. We went from 7 significant figures to 3 significant figures.
The most basic rounding rule is to drop the least significant figure (one furthest to the right) if it is less than 5.
2.2= 2 (rounding from 2 to 1 significant figure )
1.4 = 3 (rounding from 2 to 1 significant figure )
If the least significant figure is greater than 5, you add to the digit to the left of it.
1.67 = 1.7 (rounding from 3 to 2 significant figures )
1.6 = 2 (rounding from 2 to 1 significant figure )
If the rounding digit is '5' you have a few options, but for elementary school you can just round up too.
2.25 = 2.3 (rounding from 3 to 2 significant figures )
1.2418 = 1.242 (rounding from 5 to 4 significant figures )
8.5 = 9 (rounding from 2 to 1 significant figure )
95 = 100 (rounding from 2 to 1 significant figure )
125 = 130 (rounding from 3 to 2 significant figures )
A more fancy rule when you have a '5' to deal with uses the odd/even rule: If the digit immediately to the right of the last significant figure is equal to 5, you round up if the last significant figure is odd. You round down if the last significant figure is even.
24.35 = 24.4 round up, since the '3' is odd.
24.25 = 24.2 round down, since '2' is even.
You don't need this fancy rule and can just always round up when you have a '5' as the least significant figure, but if you want to be a rounding expert, it is good to know, since it gives the best results.
