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I am not sure what this sentence means? It is like there is a number which is within 2 but also as a multiple of 10? Sorry, this must be incorrect. Thanks!

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This probably refers to numbers compared to integer multiples of 10. Those are 10, 20, 30, 40, etc.

The number 31 is 1 away from 30, a multiple of 10. That number is within a distance of 2 from the nearest multiple of 10, so it meets the criterion.

The number 36 is 4 away from 40, the nearest multiple of 10, so it does not meet the criterion, because 4 is greater than 2.

So, subtract the number in question from the nearest multiple of 10, and compare the difference to 2. If it's greater than 2, it is not within 2 of a multiple of 10. If the difference is less than 2, it is within 2 of a multiple of 10.

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Well it is clear enough what it literally means.

10 is a multiple of 10, so in that case, it means somewhere between 8 and 12, for a range of relative error of plus or minus 20%.

20 is a multiple of 10, so in that case it means somewhere between 18 and 22, for a range of relative error of plus or minus 10%.

10,000 is a multiple of 10, so in that case it means somewhere between 9998 and 10002, so in that case it means a range of error of plus or minus 0.02%.

In other words, it is a perfectly clear statement. It is also perfectly useless.

EDIT Among the mathematically aware, a "multiple" of a number almost always means an integer multiple of that number. Otherwise the phrase is utterly meaningless because every real number imaginable is a multiple of every other real number except zero.

If p and q are real numbers and q is not zero, then p/q is a real number and p is a multiple of q because (p/q) * q = p. If you do not interpret "multiple" as integer multiple, then the phrase "within 2 of a multiple of ten" simply means number.

SECOND EDIT The comments below have persuaded me to expand my answer.

The "within 2 of x" means anywhere from x minus 2 through x plus 2 inclusively. So "within 2 of 100" simply means somewhere from 98 through 102. The meaning is clear. Moreover such phrases may be of practical use. The specific one that confused you has a clear meaning; it can be defined mathematically, but it is impossible for me to see in what context outside of a classroom it would ever arise. This lack of any connection to a practical context may have contributed to your difficulty in interpreting its meaning.

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    What the clause literally means can be useful, in a relevant context. One obvious relevant context would be in a programming assignment: "Write a routine to check whether a number is within 2 of a multiple of 10." Jun 2, 2020 at 5:18
  • And the practical use of such a routine would be what exactly? Jun 2, 2020 at 12:57
  • Passing the class, exploring alternate implementations, perhaps getting a decent golf score. Basic coding tutorials are rife with such things. Jun 2, 2020 at 13:59
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    Anyway, I'm not making my point clear. Shame on me and let me start again. Pretend OP said something like "My friends live in different cities. I suggested we meet at Benihana, but they said they'd only consider places within a couple miles of somebody's home. What could that mean? Can a place be within two miles but also in several separate cities? It doesn't make sense together." I don't think a good answer would include "It is a perfectly clear statement. It is also perfectly useless." The grammar and semantics here are useful to know in everyday plain English. Jun 2, 2020 at 20:23
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    OK. I agree that statements similar in structure to the one given may arise in a practical non-pegagogical setting. I shall make an edit. Jun 2, 2020 at 21:50

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