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A round number is mathematically defined as the product of a considerable number of comparatively small factors as compared to its neighbouring numbers, such as 24 = 2*2*2*3 (4 factors, as opposed to 3 factors for 27; 2 factors for 21, 22, 25, and 26; and 1 factor for 23).
A round number is informally considered to be an integer that ends with one or more zeros. So, 590 is rounder than 592, but 590 is less round than 600. In both technical and informal language, a round number is often interpreted to stand for a value or values near to the nominal value expressed. For instance, a round number such as 600 might be used to refer to a value whose magnitude is actually 592, because the actual value is more cumbersome to express exactly. Likewise, a round number may be used to refer to a range of values near the nominal value that expresses imprecision about a quantity. Thus, a value reported as 600 might actually represent any value near 600, possibly as low as 550 or as high as 650, all of which would round to 600.
A number ending in the digit 5 is also considered more round than one ending in another non-zero digit. For example, the number 25 tends to be seen as more round than 24. Thus someone might say, upon turning 45, that their age is more round than when they turn 44 or 46. These notions of roundness are also often applied to decimal numbers, so 2.3 is rounder than 2.297, because 2.3 can be written as 2.300.
Numbers can also be considered "round" in numbering systems other than decimal (base 10). For example, the number 1024 would not be considered round in decimal, but the same number ends with a zero in several other numbering systems including binary (base 2: 10000000000), octal (base 8: 2000), and hexadecimal (base 16: 400).
Psychology and sociology
Psychologically round numbers form waypoints in pricing and negotiation. So starting salaries are usually round numbers. Prices are often pitched just below round numbers in order to avoid breaking such a psychological barrier.
Round-number anniversaries are often especially celebrated. For example, a fiftieth birthday, the centenary of an event, or the millionth visitor or customer to a location or business.
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