Rounding

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Rounding of a number means replacing the numeral by another numeral that has fewer significant figures. The decision whether to round up or down depends on the value of the leading digit being rounded off. The convention is that 0, 1, 2, 3, and 4 are effectively just chopped off (truncated) while 5, 6, 7, 8, and 9 are truncated but the first digit not truncated is increased by a value of one.
Suppose that, measuring to the nearest millimetre we recorded some measurements as (in metres) 52.365, 12.764, 4.986, 2.031, and 5.699. If we decided to change them to measurements to the nearest centimet
re (that is, to two decimal places (2dp)) we could record them as 52.37, 12.76, 4.99, 2.03, and 5.70.
Using a metre ruler graduated in millimetres can help with an understanding of these roundings. The numbers 52.365 etc. above are expressed to three decimal places (3dp) while the numbers 52.37 etc above are expressed to two decimal places (2dp).