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I want to make a question having an answer as follows:

5 is the third prime number.

The bold part is the answer. How to phrase the question?

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We need a [jeopardy] tag... –  advs89 Mar 7 '11 at 20:00
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An eternal question about English. :-) We all heard this growing up. (There are expressions in our native languages, equivalent to "how many'th" but grammatical, so this was a natural question to ask.) –  ShreevatsaR Mar 8 '11 at 9:32
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14 Answers

There is no single, definite, one or two-word answer to that. There is some usage, mostly oral, of constructs like “5 is the how manyth prime number?”, but it is definitely not Standard English.

So, the answer to your question will be to reformulate it. For example, if it were a question to a math test, I would say:

For the sentence “5 is the nth prime number” to be correct, what should be the value of n?

or

5 is the nth prime number. What is the correct value of n?

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Which one is the correct one? what should be the value of n or what should the value of n be? –  LaTeX Feb 22 '11 at 10:21
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I think both are correct: what should be X? and what should X be? (for X = “the value of n”). However, the longer X is, the clearer the first construct is. –  F'x Feb 22 '11 at 10:26
    
Against 20 upvotes, I hesitate to ask, but I wonder how this answer can be correct. The value of n in the examples here would be 5, which is not an ordinal number, and the OP asks for a question form that would require an ordinal number as the answer. No? What am I missing? –  sarah Mar 4 at 2:33
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What is the ordinal status of 5 in the set of prime numbers?

sounds a bit too mathematical, huh?

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Q: How many prime numbers precede the number 5?
A: 2

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+1 for you @crowne. –  LaTeX Mar 10 '11 at 8:33
    
The OP is looking for a way to prompt an answer in ordinal form. "2" is not in ordinal form. "Second" or "third" is. –  dj18 Oct 15 '12 at 14:00
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What is the position of n in the series of prime numbers?

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I would phrase it as:

In a list of prime numbers, in which position does 5 appear?

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How many prime numbers are less than five? The answer sought will be one greater than the answer given.

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But the answer will not be given in ordinal form. –  dj18 Oct 15 '12 at 13:58
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In the series of primes described using the following constructs, 2 is the first prime number and 3 is the second prime number, what is 5?

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Alternatively, "If two is described as 'the first prime number', and three is described as 'the second prime number', how would five be described?" –  supercat Oct 15 '12 at 17:59
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Rephrasing slightly, I'd ask: what is the position of 5 in the sequence of prime numbers?

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It can be "What is the rank of 5 in prime number series?"

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Why not simply use the term ordinal directly? For example:

What ordinal number reflects the position of the number five in the set of prime numbers?

or more succinctly:

What is the ordinality of five in the set of prime numbers?

Ordinality might be a bit of a neologism, but the meaning should be clear to anyone familiar with the root, I think.

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Among others, I like this answer most. –  LaTeX Mar 4 '11 at 0:57
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Strictly correct, but not a very easy question to understand if by people who are not well educated in mathematics. (Most people I know who don't do science for a living will stop listening to what you say at “ordinal number”, and start thinking hard about what it could possibly mean.) –  F'x Mar 4 '11 at 7:59
    
Yeah. This is probably the best way to answer the question followed by FX's second suggestion. –  Dark Star1 Mar 9 '11 at 16:58
    
@F'x Since this is a math question (it is asking about Ordinal and Prime numbers), how can it be inappropriate to expect knowledge of math to be required to answer it correctly? I guess we are broadening the question to the generic, asking how to get an ordinal number as an answer to any question, math related or not? I'm thinking then, that the question should be edited to reflect this. I'd do it, but not being a regular here, though, I'm not sure I should. –  sarah Mar 4 at 2:43
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FX's answer is an excellent option (and has my vote).

One other technique that is sometimes used in math or science questions is to give an example response as part of the question:

The number two is the first prime number. In the sequence of prime numbers, what is the position of the number five?

This is particularly effective for a verbal question, where using a variable 'n' may be more confusing than it is in print (depending on the audience). One disadvantage is the relative verbosity of this form, but it is mathematically unambiguous while expressly stating the desired form of the answer.

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I think you could say:

5 is which prime number?

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An my answer would be, "The one before seven." –  Peter Olson Mar 3 '11 at 17:26
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And I'd say, "Don't be a smartass." :-) –  Hellion Mar 4 '11 at 4:44
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Without more context, I would not know what form of answer you were looking for. –  luqui Mar 7 '11 at 7:51
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I agree with luqui. If I saw "5 is which prime number?" without context, I'd probably say "Er… 5 is 5. Which other prime number can it be?" –  ShreevatsaR Mar 8 '11 at 16:38
    
This could also be answered by "5 is the happy prime number!" Or any other adjective. Not necessarily indicating the position. –  mikhailcazi Oct 12 '13 at 11:24
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The phrase n...nth is conventionally used for cases like this. In one sentence, the question can concisely be phrased thus:

For what value of n is five the nth prime number?

If you wanted to use words that are not coined by math, you could use a slightly more ambiguous question:

Which term is five on the series of prime numbers?

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Whew, I had to read this a few times

In a list of prime numbers, where is the number 5?

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The question is asked the other way around (5 is not the answer, but part of the question). –  F'x Feb 22 '11 at 10:15
    
Also, 5 is the third prime number, starting from 2 (1 is usually not considered prime, and if it is, then 5 is the fourth prime number starting from 1). –  F'x Feb 22 '11 at 10:16
    
Right you are. I had to read the question again –  mplungjan Feb 22 '11 at 10:17
    
Jedi you are; in you is the Force. –  kiamlaluno Mar 8 '11 at 0:43
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