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Rounding is the process by which a number is written correct to the nearest whole number or a given number of decimal places. Useful when estimating answers.
The rule for rounding up or down depends on the first digit NOT used in the answer, if it is five or more - round up, if four or less - round down
1.845 1.9 2
18.45 18 20
184.5 185 200
1845 1800 2000

To remove the last few digits of a decimal; to change a number into the whole number nearest to it, or the tenth which is nearest to it, etc.
We often do this if we think that the last few digits of a decimal are not really accurate; we throw them away so as not to give a false impression of how accurate our result is.