You may also like

Chocolate

There are three tables in a room with blocks of chocolate on each. Where would be the best place for each child in the class to sit if they came in one at a time?

Four Triangles Puzzle

Cut four triangles from a square as shown in the picture. How many different shapes can you make by fitting the four triangles back together?

Cut it Out

Can you dissect an equilateral triangle into 6 smaller ones? What number of smaller equilateral triangles is it NOT possible to dissect a larger equilateral triangle into?

Multiples Grid

Age 7 to 11
Challenge Level

Here is a $100$ grid with some numbers shaded:

multiples of 5 and 7

What do all the numbers shaded blue have in common?
What do you notice about all the numbers shaded pink?
Can you work out why two of the numbers are shaded in a maroon colour?

Now, here is part of a $100$ square shaded in a different way:

shaded pink: 24, 26, 34, 36, 44, 46 shaded blue: 35

Can you explain the shading this time?

Here are some more parts of the $100$ square, each one shaded according to different rules. Can you work out what the rules are for each?
Is there only one solution each time?

shaded blue: 66 and 77; shaded maroon: 88

shaded blue: 45, shaded maroon: 36 and 54

shaded pink: 16; shaded blue: 27

This problem is featured in Maths Trails - Excel in Problem Solving, one of the books in the Maths Trails series written by members of the NRICH Team and published by Cambridge University Press. For more details about the other books in the series, please see our publications page .


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.
Copyright © 1997 - 2023. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project.