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Original poster

Maths question - Civil Service is wrong (we now have 100% more threads about the subject)

434 replies

Sharingaroomtinightthen · 02/08/2025 13:36

When I posted late last night I thought I’d get maybe half a dozen replies confirming the question didn’t have the correct answer and advising whether to tell the Civil Service recruiters. But here we are 1000 posts later.

https://www.mumsnet.com/talk/am_i_being_unreasonable/5384347-maths-test-to-think-civil-service-have-it-wrong

Maths question - Civil Service is wrong (we now have 100% more threads about the subject)
OP posts:
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7
Nchangeo · 07/08/2025 00:05

KrisAkabusi · 06/08/2025 23:56

No it isn't. They are asking for the increase, not the final value.

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Which is not an option in the answers. It’s oddly worded but the maths adds up and can be back tested.

It’s not any of the options over 300. It’s not 150. Logic deduces 225.
Backtest OPs original 200 to 450 is 200 x 2.25 =450. Done. Tick box move on.

It’s like VAT. That’s 20%. You want to work it out it’s x 1.2 (120%).

Merryoldgoat · 07/08/2025 00:03

Nchangeo · 06/08/2025 23:49

It’s 225%

Oh FFS.

KrisAkabusi · 06/08/2025 23:56

Nchangeo · 06/08/2025 23:49

It’s 225%

No it isn't. They are asking for the increase, not the final value.

Nchangeo · 06/08/2025 23:49

It’s 225%

Team125 · 06/08/2025 23:39

Merryoldgoat · 06/08/2025 16:25

@Team125

It’s YOU! You took the best name ever 🤣

Hahaha! I couldn’t resist the suggestion in the earlier thread 🤣

Horsie · 06/08/2025 22:19

I would just like to say that I was immensely tired when I posted on that other thread and I am not nearly as thick as I made myself sound!

BrickBiscuit · 06/08/2025 22:19

poetryandwine · 06/08/2025 22:18

Okay, subtraction - same comment devil and about 2nd bucket! Til tomorrow.

Same thing though - multiplication is just repeated addition, and division ...

poetryandwine · 06/08/2025 22:18

Okay, subtraction - same comment devil and about 2nd bucket! Til tomorrow.

BrickBiscuit · 06/08/2025 22:17

poetryandwine · 06/08/2025 22:15

Okay, I am a mere maths orientated STEM PhD but moderately theoretical. I don’t get this. Infinite divisibility is the devil’s handiwork, as anything infinite is technically approached as a limit.

So I may have a hard time with an ‘infinite vibe’. I see that intuitively the first bucket will never empty. I don’t see that you have .999999…….. of the particles in the second?

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Isn’t it infinite subtraction, rather than divisibility?

poetryandwine · 06/08/2025 22:16

PS Probably signing off MN now. No disrespect to this tantalising problem

poetryandwine · 06/08/2025 22:15

BrickBiscuit · 06/08/2025 22:03

You have a bucket full of a sand-like substance, and an empty bucket. The substance is infinitely divisible, so there is no particle too big to separate. You transfer 90% from the full to the empty bucket, and repeat ad infinitum. Your ‘new’ bucket will contain 0.999… of the substance, but will never be full.

Edited
Show quote history

Okay, I am a mere maths orientated STEM PhD but moderately theoretical. I don’t get this. Infinite divisibility is the devil’s handiwork, as anything infinite is technically approached as a limit.

So I may have a hard time with an ‘infinite vibe’. I see that intuitively the first bucket will never empty. I don’t see that you have .999999…….. of the particles in the second?

BrickBiscuit · 06/08/2025 22:03

poetryandwine · 06/08/2025 21:47

What thought experiment is this?

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You have a bucket full of a sand-like substance, and an empty bucket. The substance is infinitely divisible, so there is no particle too big to separate. You transfer 90% from the full to the empty bucket, and repeat ad infinitum. Your ‘new’ bucket will contain 0.999… of the substance, but will never be full.

OneDearWasp · 06/08/2025 21:57

Isn't it 225%?

END of year one say 100.

Trebles to be 300.

Loses one quarter to make 225.

poetryandwine · 06/08/2025 21:47

BrickBiscuit · 06/08/2025 21:09

Thank you, very kind. One thing I am not is a mathematician. I have published peer-reviewed quantitative research, but always employing a statistician. I could not, for example, accept that 0.999... =1 from a notational proof while a thought experiment involving infinitely-divisible particles shows otherwise. This probably disbars me from the field. So, we have two well-posed problems, the MHP and this MCQ. Each gives sufficient information to return a unique solution which varies with the data provided. No further assumptions are necessary. The actual calculations are trivial. Yet knowledgeable people argue passionately for the wrong answer. The paradox in both cases is that the solution is simple, yet counter-intuitive to both novices and experts.

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What thought experiment is this?

BrickBiscuit · 06/08/2025 21:20

boobleblingo · 05/08/2025 20:02

The Monty Hall problem is a beautiful example of how probabilities can be counterintuitive.

This is an example of a poorly worded question - poorly worded because, while the question is unambiguous, the correct answer is not provided so we can't be sure that the setter understood the words they were using, like so many on this thread.

Not remotely the same thing.

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I see the MCQ question working on two levels. First, yes, the correct answer is omitted from the options. Faced with this error, people have to decide how to answer. Second, the actual calculation (which is trivial in its simplicity) is independent of the option set. Many people are doing it wrong. That’s the counter-intuitive bit, and I think that mirrors the MHP quite closely.

BrickBiscuit · 06/08/2025 21:09

poetryandwine · 06/08/2025 11:05

I’m thinking you aren’t the ‘naive thicko’ - I know that isn’t quite an accurate quotation, but it is close, and captures the spirit - that you first described yourself to be. :)

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Thank you, very kind. One thing I am not is a mathematician. I have published peer-reviewed quantitative research, but always employing a statistician. I could not, for example, accept that 0.999... =1 from a notational proof while a thought experiment involving infinitely-divisible particles shows otherwise. This probably disbars me from the field. So, we have two well-posed problems, the MHP and this MCQ. Each gives sufficient information to return a unique solution which varies with the data provided. No further assumptions are necessary. The actual calculations are trivial. Yet knowledgeable people argue passionately for the wrong answer. The paradox in both cases is that the solution is simple, yet counter-intuitive to both novices and experts.

Original poster
Sharingaroomtinightthen · 06/08/2025 19:25

SerendipityJane · 06/08/2025 11:10

I have repeatedly pointed out that without the candidate guidance, we are at a disadvantage.

One way to move forwards is to invent our own reality, and then answer that - and it's not impossible this is actually what is required. After all it's looking for the ideal candidate for HMRC - never known for fans of reality.

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It’s just your bog standard candidate guidance.

https://www.gov.uk/guidance/using-the-civil-service-jobs-website

OP posts:
Merryoldgoat · 06/08/2025 16:25

@Team125

It’s YOU! You took the best name ever 🤣

Team125 · 06/08/2025 12:58

BrickBiscuit · 05/08/2025 14:10

Yes, I showed them the photo in the OP. They didn’t reach a conclusion, but gave a couple of answers at first sight and looked for a pen. I think writing the algebra down would have solved it. Perhaps they couldn’t see the wood for the trees (and we had had a couple of beers already). Though I got it at first sight, in my head, and I’m thick.

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Civil service tests do not require algebra, trust me 🤣

Sometimes these ever so intelligent academics actually have no common sense or reasoning skills and look for complexities that aren’t there. That’s why these tests are designed not just to test for the best mathematician, but the best all round person for the role.

SerendipityJane · 06/08/2025 11:10

jcyclops · 06/08/2025 00:09

It could be a VERY clever (sneaky) question that deliberately has no correct option for the answer. It would be especially useful in a timed test - OP doesn't say if the test was timed. If you take too long on the question you could lose marks by not completing the whole test.

If I met this in a test I would NOT tick any of the incorrect boxes (instant mark lost). I would write 125% to the side and MOVE ON.

PS. I should have changed my username to Kobayashi Maru for this post.

I have repeatedly pointed out that without the candidate guidance, we are at a disadvantage.

One way to move forwards is to invent our own reality, and then answer that - and it's not impossible this is actually what is required. After all it's looking for the ideal candidate for HMRC - never known for fans of reality.

poetryandwine · 06/08/2025 11:05

BrickBiscuit · 06/08/2025 10:17

And the Monty Hall Problem is still causing difficulties. Although the solution is settled (it's 'Yes'), the expert view that, for example, the assumption that 'the host never varies their behaviour' is essential is false. It's also arguably 7/12, rather than 2/3.

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I’m thinking you aren’t the ‘naive thicko’ - I know that isn’t quite an accurate quotation, but it is close, and captures the spirit - that you first described yourself to be. :)

CyberStrider · 06/08/2025 10:50

The original question is simple algebra. Nothing to do with Monty Hall,statistics or variable change!

irregularegular · 06/08/2025 10:20

@BrickBiscuit It is interesting how few difficulties it caused me as a naive 17 year old.

BrickBiscuit · 06/08/2025 10:17

irregularegular · 06/08/2025 10:09

I love the Monty Hall problem. I think I largely owe my Oxford undergraduate place to it. Back in the day when there were written entrance tests I took the "Maths for non-mathematicians" paper and one of the problems was the Monty Hall problem, which I had never come across before and solved perfectly using Bayes rule. I felt very smug when I later learned it was a problem that had caused some mathematicians so many difficulties.

(The rest of the paper was a bit a car crash! It wasn't the kind of paper where you were expected to be able to do everything, but I didn't know that at the time and I came out thinking it was a complete and utter disaster!)

Edited to add: I just checked the dates. Apparently the Monty Hall problem became well known in 1990 after it appeared in Marilyn vos Savant's "Ask Marilyn" column in Parade magazine in 1990. I took the entrance exam in 1989 so it was reasonable to belive no candidates would have seen it before.

Edited

And the Monty Hall Problem is still causing difficulties. Although the solution is settled (it's 'Yes'), the expert view that, for example, the assumption that 'the host never varies their behaviour' is essential is false. It's also arguably 7/12, rather than 2/3.

irregularegular · 06/08/2025 10:16

jcyclops · 06/08/2025 00:09

It could be a VERY clever (sneaky) question that deliberately has no correct option for the answer. It would be especially useful in a timed test - OP doesn't say if the test was timed. If you take too long on the question you could lose marks by not completing the whole test.

If I met this in a test I would NOT tick any of the incorrect boxes (instant mark lost). I would write 125% to the side and MOVE ON.

PS. I should have changed my username to Kobayashi Maru for this post.

Are there marks deducted for incorrect answers in the test? Sorry I can't be bothered to read the whole thread to see if it says this somewhere.

If so, then better to skip I guess.

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