Skip over navigation
Cambridge University Faculty of Mathematics NRich logo
menu search
  • Teachers expand_more
    • Early years
    • Primary
    • Secondary
    • Post-16
    • Events
    • Professional development
  • Students expand_more
    • Primary
    • Secondary
    • Post-16
  • Parents expand_more
    • Early Years
    • Primary
    • Secondary
    • Post-16
  • Problem-Solving Schools
  • About NRICH expand_more
    • About us
    • Impact stories
    • Support us
    • Our funders
    • Contact us
  • search

Or search by topic

Number and algebra

  • The Number System and Place Value
  • Calculations and Numerical Methods
  • Fractions, Decimals, Percentages, Ratio and Proportion
  • Properties of Numbers
  • Patterns, Sequences and Structure
  • Algebraic expressions, equations and formulae
  • Coordinates, Functions and Graphs

Geometry and measure

  • Angles, Polygons, and Geometrical Proof
  • 3D Geometry, Shape and Space
  • Measuring and calculating with units
  • Transformations and constructions
  • Pythagoras and Trigonometry
  • Vectors and Matrices

Probability and statistics

  • Handling, Processing and Representing Data
  • Probability

Working mathematically

  • Thinking mathematically
  • Developing positive attitudes
  • Cross-curricular contexts

Advanced mathematics

  • Decision Mathematics and Combinatorics
  • Advanced Probability and Statistics
  • Mechanics
  • Calculus

For younger learners

  • Early Years Foundation Stage

What Number?

Age 5 to 7
ShortChallenge Level Yellow starYellow star
  • Problem
  • Getting Started
  • Solutions
  • Teachers' Resources
We had just a few correct solutions and these ones came from from Pakuranga Heights Primary School in New Zealand. First Blaize and Abhishek and then one from Grace.

Blaize, 1+1=2
Abhishek, 12+12=24

Grace, if the number is under 25 and it adds up to an even number, but the ones digit is twice the tens digit then when you get it, it will be easier. Here is a clue I'm giving you. The ones digit is 4, so what is half of four and then you'll get your answer.


Katarina, and Syed from Foxford School and Community College, both used the same method. Syed says:

To solve this problem I go through one clue at a time and use my elimination skill to find out what the number is:

Clue 1 : 'I am less than 25'
So from this clue I list all the numbers below 25.
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24

Clue 2 : 'My unit digit is twice my tens digit'
This clue can be split into smaller clues:

'It has a tens and unit digit'
So now I get rid of all the numbers from the list from clue 1 that don't apply to this clue. So I have:
10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24

'Its unit digit is twice its tens digit'
12, 24

Clue 3: 'My digits add up to an even number'
1 + 2 = 3 which not an even number so that it can only be 24


Alexis approached the problem slightly differently writing:

Since the ones digit has to be twice the tens, I made a list of those possible numbers: 12, 24, 36, 48  ...
But it has to be less than 25: 12 and 24
Adding the digits has to result in an even number: 1+2=3, 2+4=6
6 is an even number so 24 is the answer!


Thank you for all these contributions and explanations to show your thinking.

Related Collections

You may also like

Writing Digits

Lee was writing all the counting numbers from 1 to 20. She stopped for a rest after writing seventeen digits. What was the last number she wrote?

Number Detective

Follow the clues to find the mystery number.

Palindromes

Find out about palindromic numbers by reading this article.

  • Tech help
  • Accessibility Statement
  • Sign up to our newsletter
  • Twitter X logo

The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

University of Cambridge logo NRICH logo