These problems invite you to find all possible solutions. One of the best answers to "How do you know you have found them all" is to be able to say "I worked systematically!"
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problem
Shady symmetry
How many different symmetrical shapes can you make by shading triangles or squares?
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problem
Isosceles triangles
Draw some isosceles triangles with an area of $9$cm$^2$ and a vertex at (20,20). If all the vertices must have whole number coordinates, how many is it possible to draw?
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problem
M, M and M
If you are given the mean, median and mode of five positive whole numbers, can you find the numbers?
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problem
Peaches today, peaches tomorrow...
A monkey with peaches, keeps a fraction of them each day, gives the rest away, and then eats one. How long can his peaches last?