### Number Detective

Follow the clues to find the mystery number.

### A-maze-ing

Did you know that ancient traditional mazes often tell a story? Remembering the story helps you to draw the maze.

### The Naked Pair in Sudoku

A particular technique for solving Sudoku puzzles, known as "naked pair", is explained in this easy-to-read article.

# Which Numbers? (2)

##### Stage: 2 Challenge Level:

This problem is similar to Which Numbers? (1), but slightly more tricky. You may like to try that one first.

I am thinking of three sets of numbers less than $101$. They are the blue set, the red set and the black set.
Can you find all the numbers in each set from these clues?

These numbers are some of the blue set: $26, 39, 65, 91$, but there are others too.
These numbers are some of the red set: $12, 18, 30, 42, 66, 78, 84$, but there are others too.
These numbers are some of the black set: $14, 17, 33, 38, 51, 57, 74, 79, 94, 99$, but there are others too.

There are sixteen numbers altogether in the red set, seven numbers in the blue set and fifty numbers in the black set.

These numbers are some that are in just one of the sets: $6,10, 15, 24, 33, 48, 56, 65, 75, 93$, but there are others too.
These numbers are some that are in two of the sets: $12, 13, 36, 54, 72, 96$, but there are others too.
The only number in all three of the sets is $78$.
These numbers are some that are not in any of the sets: $5, 8, 22, 27, 44, 49, 63, 68, 82, 86, 100$, but there are others too.
There are twelve numbers that are in two of the sets.
There are $41$ numbers that are not in any of the sets.

You can download a sheet of all this information that can be cut up into cards.

Can you find the rest of the numbers in the three sets?
Can you give a name to the sets you have found?

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. More information on many of our other activities can be found here.
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