# How should I phrase a question that must be answered with an ordinal number (e.g., the third prime)?

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?

• We need a [jeopardy] tag...
Mar 7, 2011 at 20:00
• 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.) Mar 8, 2011 at 9:32
• If George Washington was the first president, which number is Barack Obama? ... and maybe include instructions on whether to count Grover Cleveland once or twice. Mar 17, 2014 at 15:24
• English doesn't have a special question word, nor even a good construction, to ask questions specifically about ordinal numbers (first, second, ..., forty-fourth, ...), the way How many? asks about cardinal numbers. If we had a productive morphology, we could ask "*How manyth President is Barack Obama?" But we don't. Mar 17, 2014 at 15:38
• Possible duplicate of Framing a question whose answer is an ordinal number May 27, 2016 at 15:42

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.

• I'm curious, why doesn't English have a word for this? Seems a pretty common useful word in our daily lives. Does this problem exist in all Indo-European languages? Jan 20, 2021 at 11:16
• @WeirdElfB0y No, e.g. German has "Die wievielte Primzahl ist 5?" -- Usefulness and existence of words is not always as correlated as one would wish. English has no single word for the useful concept "the day after tomorrow" and uses exactly that phrase instead, but it does have a single word for the less useful concept "to throw somebody out of the window" Jun 30, 2021 at 0:10
• I'm curious, what the word "to throw somebody out of the window"? May 9 at 6:33

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.

• Among others, I like this answer most. Mar 4, 2011 at 0:57
• 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, 2011 at 7:59
• Yeah. This is probably the best way to answer the question followed by FX's second suggestion. Mar 9, 2011 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. Mar 4, 2014 at 2:43

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?

• Which one is the correct one? `what should be the value of n` or `what should the value of n be`? Feb 22, 2011 at 10:21
• 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, 2011 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? Mar 4, 2014 at 2:33
• @sarah "nth" converts the cardinal `n` to the ordinal `nth`. Oct 20, 2015 at 23:22

Rephrasing slightly, I'd ask: what is the position of 5 in the sequence of prime numbers?

You could use sequentially, as in

Sequentially, which prime is 5?

However, the term is not completely unambiguous: "Sequentially, which president was Abraham Lincoln?" could legitimately be answered with "Well, he was the one after James Buchanan and before Andrew Johnson"; similarly, as a prime, five is "preceded by 3 and succeeded by 7".

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

I would phrase it as:

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

I think you could say:

5 is which prime number?

• An my answer would be, "The one before seven." Mar 3, 2011 at 17:26
• Without more context, I would not know what form of answer you were looking for. Mar 7, 2011 at 7:51
• 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?" Mar 8, 2011 at 16:38
• This could also be answered by "5 is the happy prime number!" Or any other adjective. Not necessarily indicating the position. Oct 12, 2013 at 11:24
• I must join the chorus--the question this answer proposes has too many potential answers and certainly does not lead invariably, or even most frequently, to the sort of answer required by the OP> Mar 4, 2014 at 2:31

What is the position of n in the series of prime numbers?

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?

• Alternatively, "If two is described as 'the first prime number', and three is described as 'the second prime number', how would five be described?" Oct 15, 2012 at 17:59

In getting an ordinal response from our kids on quizzes we ask "what is the number-[thing]?", pronounced almost as if it were hyphenated "what number-president is G. W. Bush?". In writing I would be very specific, usually with leading example: "In terms of land area Alaska is first; what is Rhode Island?"

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

• The question is asked the other way around (5 is not the answer, but part of the question).
– F'x
Feb 22, 2011 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, 2011 at 10:16
• Right you are. I had to read the question again Feb 22, 2011 at 10:17
• Jedi you are; in you is the Force. Mar 8, 2011 at 0:43

What is the ordinal status of 5 in the set of prime numbers?

sounds a bit too mathematical, huh?

The answer nobody gave is because they assume the answer must be given as a sentence. You will want to ask a question that fills in the blank.

The question is usually posed in tests as:

Foo is the ____(st/nd/rd/th) bar.

Either you want the ordinal or you want what the ordinal counts. You can't (effectively) ask for both things without some context. There are many answers to 5 is __. There's only one answer for 5 is the _(st/nd/rd/th) prime number

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?

Who is the 44th President of the United States?

Otherwise, the possibilities are infinite.

How many presidents have there been up to the present day, and who is the most recent?

Why is Barack Obama #44 on a list of US Presidents?

You're going to need to narrow down the context.

For non-mathematical contexts (e.g. here), phrasing the question as a single sentence that should yield a definitive correct answer ending end with one of "-st,","-nd",or"-th", you could pose the inquiry as

Which numberth {category} is [object]?

; for the mathematical case, you could phrase it as

What is the ordinality of [object] in the {plural of category}?

. Although neither "numberth" nor "ordinality" are widespreadly recognized words (the former less-so than the latter), they nonambiguously convey succinctly the concept for the respetive purpose desired better, I think, than usable alternative phrasings.

The prime number (N) is equal to the sum total of all prime numbers < N. Which prime number is N in the ordinal ranking of primes?

That's if you want to be a bit mean and confusing.