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Posts from the ‘arithmetic’ Category


June 6th, 2011



Choose any 4 digit number except for 1111, 2222, 3333, 4444. I’ll choose 1502.

Rearrange the digits to give you the biggest and the smallest numbers you can. 5210 and 0125.

Find the difference of these two numbers. 5210 – 0125 = 4995.

Repeat for the new number you get (using zeros to supplement any missing digits if necessary; you must always have 4). 9954 – 4599 = 5335.

Keep repeating until you have a good reason to stop.


5210 – 0125 = 4995
5533 – 3355 = 2178
8712 – 1278 = 7434
7443-3447 = 3996
9963 – 3699 = 6264
6642 – 2466 = 4176
7641 – 1467 = 6174

I’m not going to repeat any more because 6174 => 7641 – 1467 which is the calculation I just did!


  • So every number we’ve tested came to 6174. Do you think all numbers will come to 6174? Why? In maths we can’t say we’re sure about something unless we have a good reason, so unless you want to go through every number and check it, we can’t say that we know that every number sequence we choose will go to 6174! (This is how mathematics differs from science, we don’t just make hypotheses and wait until the next time they break, we find ways to be certain!)
  • Would we have to check every different number to be certain or are there shortcuts we can take? (should we check both 1234 and 1243 separately?)
  • My rules stipulated that you couldn’t choose all the same number: 1111 or 2222. What would happen if you did use those numbers?
  • What about two digit numbers or three digit numbers?
  • Could we frame the problem as (1000a + 100b + 10c + d) – (1000d + 100c + 10b + a)?
  • What about five or six digit numbers (or more…)? You may need a computer to help!


I just came across this interesting bit of mathematics via Twitter: It offers the opportunity for a beautiful open-ended task for secondary maths classes of all abilities. The initial task is easy and produces a startling result that feels like a trick; that hook can lead to discussion and further work about probability, permutations, algebra and programming.

Maths Education and the Metric System

September 7th, 2008


The following conversation in Metric Views catches the attention both for the interesting article and the subsequent comments.

Metric Views: Are our schools entrenching the ‘very British mess’?

The gist is that our schools reflect our current social muddle by teaching both imperial and metric measures and their relative magnitudes in school. In the article it is argued that the time and cost wasted on this is horrifying.

I have no love of imperial measures; I find it frustrating to have to remember how many pounds are in a stone, or ounces in a pound, or yards in a mile, and struggle to do so. I also find it difficult to convert between anything other than kilometers and miles. I know my weight in stone, but not in pounds, and certainly not in kilograms. I know my height in both metres and feet-and-inches. I am not sure that I can estimate volume in any unit with any degree of accuracy. It’s a horrible, muddy, confusing mess; that is undeniable.

I think my misgivings about the article are about the underlying idea that we should stop teaching both measures to achieve a feat of social engineering; by removing from the minds of the youth any conception of imperial measures, we would hasten the demise of imperial measures, which would be a Good Thing.

My difficulty is that feet-and-inches is such a good measure of height. I am 1.83m or 183cm, but neither is as satisfying as being 6′ tall, and neither is immediately conjourable in my mind. I don’t like Americans’ removal of ‘stone’ as a measurement either; 13 (and a bit) stone is much easier to remember than… whatever number of kilograms or pounds I am.

Feet, inches, stones and pounds are good measures because they are useful. They give us a scale rooted in humanity and the measurement of humans, and allow us to compare ourselves with others accurately. I am not convinced that the removal of these measures in classrooms will remove their common use.

I should not be confused with someone in defence of a curriculum which monitors and assesses the knowledge of different weights and measures and their conversion. Conversion is a fairly dry arithmetical topic. However, there might be problems that involve imperial or metric measures (or even their conversion) which may contain some good mathematics. I would not want that potential to be excluded from the curriculum any more than I would want their being taught made compulsory.