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'Multiples Grid' printed from http://nrich.maths.org/
Here is a $100$ grid with some numbers shaded:
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
Now, here is part of a $100$ square shaded in a different
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
Is there only one solution each time?
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 .