Mystic Maths

I can read your mind!

PsychicThink of a two digit number,*

Reverse the digits to get another two digit number,

Subtract the smaller two digit number from the other,

Add the digits of your answer together

I know what your answer is!

(* The two digits must be different)



How did I know?

A Mathematics Lesson Starter Of The Day


Topics: Starter | Algebra | Place Value | Puzzles

  • Tom, Birmingham
  • 75-57?

    I think the answer will be a multiple of 9?
  • Paul, Coventry
  • Yes, and when you add the digits of a two digit multiple of 9 the answer is always 9
  • JM, Ysgol Friars
  • AB - BA = (Ax10+B)-(Bx10+A) = 9A-9B = 9(A-B)
  • CF, Lancshire
  • This is a great starter it is a quick way to see who can subtract without a calculator and it is a good way to demonstrate the use of algebra to generalise and prove things.
  • Amy, Carlow Ireland
  • 65-56=09 0+9=9 cool it really works.
  • Amy, Carlow Ireland
  • 65-56=09 0+9=9 cool it really works.
  • Beth Lyons And Caitlin York, Us
  • It is easy to find out how he did it all he did is sort numbers out from high and low.
  • Gianni, Milan
  • 52-25=27!
    Not every number is like that!
  • Miss Hughes, London
  • My Year 4 class worked out that by reversing the numbers it always came to a multiple of 9. I was able to spot the pattern that when you use consecutive numbers (45, 56, 67, 78, 89) the answer will always be 09, as the lower tens digit will become the units and you will always have to borrow. Can anyone help me explain why ANY number, even a prime number, will result in a multiple of 9?
  • Year 4, St Alban's Warrington
  • Our Year 4 class found this interesting. Some found it hard and some found it easy. It is a good starter for the day.
  • Nigel Martin, Havant Academy
  • Declan one of my KS4 students used 99, which does not work.
  • P6/5, Balfron Primary
  • We enjoyed this challenge but still aren't sure why the answer is always 9!
  • Mr Jones, UK
  • If the digits are x and y then one number is 10x+y and the other is 10y+x.
    When you subtract the 2 numbers you are left with 9x-9y which factorises to 9(x-y) and as x-y is greater than zero as the digits are not equal and you have subtracted the smaller number, the solution will always be a multiple of 9.
  • Mr Hulbert, Kellett School HK
  • Great starter for my Y6 class!
    We asked if three different digits would make a certain number?
    So far it has always been 18 ...
    Four digit numbers?

How did you use this starter? Can you suggest how teachers could present or develop this resource? Do you have any comments? It is always useful to receive feedback and helps make this free resource even more useful for Maths teachers anywhere in the world.
Click here to enter your comments.

If you don't have the time to provide feedback we'd really appreciate it if you could give this page a score! We are constantly improving and adding to these starters so it would be really helpful to know which ones are most useful. Simply click on a button below:

Excellent, I would like to see more like this
Good, achieved the results I required
Didn't really capture the interest of the students
Not for me! I wouldn't use this type of activity.

This starter has scored a mean of 3.6 out of 5 based on 479 votes.

Previous Day | This starter is for 14 January | Next Day



Your access to the majority of the Transum resources continues to be free but you can help support the continued growth of the website by doing your Amazon shopping using the links on this page. Below is an Amazon search box and some items chosen and recommended by Transum Mathematics to get you started.


Have you read Craig's book yet?

Craig Barton must surely be the voice of Mathematics teachers in the UK. His wonderful podcasts interviewing the industry experts have culminated in this wonderful book. As Craig says: "I genuinely believe I have never taught mathematics better, and my students have never learned more. I just wish I had known all of this twelve years ago..." more...

"How I wish I'd taught Maths" is an extraordinary and important book. Part guide to research, part memoir, part survival handbook, it’s a wonderfully accessible guide to the latest research on teaching mathematics, presented in a disarmingly honest and readable way. I know of no other book that presents as much usable research evidence on the dos and don’ts of mathematics teaching in such a clear and practical way. No matter how long you have been doing it, if you teach mathematics—from primary school to university—this book is for you." Dylan Wiliam, Emeritus Professor of Educational Assessment, UCL.

The Craig Barton Book

Casio Classwiz Calculator

There is currently a lot of talk about this new calculator being the best in its price range for use in the Maths classroom. The new ClassWiz features a high-resolution display making it easier to view numerical formulas and symbols but it isn't a graphical calculator as such (it has the capacity to draw graphs on your smart phone or tablet, via a scannable QR code and an app).

As well as basic spreadsheet mode and an equation solving feature you also get the ability to solve quadratic, cubic or quartic polynomial inequalities and the answer is given just as it should be written down, using the correct inequality symbols!

This calculator has a high-performance processor and twice the memory of previous models ensuring speedy operation and superior computational power.more...

Online Maths Shop

Laptops In Lessons

Teacher, do your students have access to computers?
Do they have iPads or Laptops in Lessons?

Whether your students each have a TabletPC, a Surface or a Mac, this activity lends itself to eLearning (Engaged Learning).

Laptops In Lessons

Here a concise URL for a version of this page without the comments.

Here is the URL which will take them to a related student activity.

Student Activity

If you like this sort of thing Mystic Mathematical Mind Reader will amaze you!

Student Activity


©1997-2020 WWW.TRANSUM.ORG